net: ftgmac100: Fix oops in probe on failure to find associated PHY

netif_napi_del() should be paired with netif_napi_add(), however no
such call takes place in ftgmac100_probe(). This triggers a NULL
pointer dereference if e.g. no PHY is found by the MDIO probe:

	 [ 2.770000] libphy: Fixed MDIO Bus: probed
	 [ 2.770000] ftgmac100 1e660000.ethernet: Generated random MAC address 66:58:c0:5a:50:b8
	 [ 2.790000] libphy: ftgmac100_mdio: probed
	 [ 2.790000] ftgmac100 1e660000.ethernet (unnamed net_device) (uninitialized): eth%d: no PHY found
	 [ 2.790000] ftgmac100 1e660000.ethernet: MII Probe failed!
	 [ 2.810000] Unable to handle kernel NULL pointer dereference at virtual address 00000004
	 [ 2.810000] pgd = 80004000
	 [ 2.810000] [00000004] *pgd=00000000
	 [ 2.810000] Internal error: Oops: 805 [#1] ARM
	 [ 2.810000] CPU: 0 PID: 1 Comm: swapper Not tainted 4.10.17-1a4df30c39cf5ee0e3d2528c409787ccbb4a672a #1
	 [ 2.810000] Hardware name: ASpeed SoC
	 [ 2.810000] task: 9e421b60 task.stack: 9e4a0000
	 [ 2.810000] PC is at netif_napi_del+0x74/0xa4
	 [ 2.810000] LR is at ftgmac100_probe+0x290/0x674
	 [ 2.810000] pc : [<80331004>] lr : [<80292b30>] psr: 60000013
	 [ 2.810000] sp : 9e4a1d70 ip : 9e4a1d88 fp : 9e4a1d84
	 [ 2.810000] r10: 9e565000 r9 : ffffffed r8 : 00000007
	 [ 2.810000] r7 : 9e565480 r6 : 9ec072c0 r5 : 00000000 r4 : 9e5654d8
	 [ 2.810000] r3 : 9e565530 r2 : 00000000 r1 : 00000000 r0 : 9e5654d8
	 [ 2.810000] Flags: nZCv IRQs on FIQs on Mode SVC_32 ISA ARM Segment user
	 [ 2.810000] Control: 00c5387d Table: 80004008 DAC: 00000055
	 [ 2.810000] Process swapper (pid: 1, stack limit = 0x9e4a0188)
	 [ 2.810000] Stack: (0x9e4a1d70 to 0x9e4a2000)
	 [ 2.810000] 1d60: 9e565000 9e549e10 9e4a1dcc 9e4a1d88
	 [ 2.810000] 1d80: 80292b30 80330f9c ffffffff 9e4a1d98 80146058 9ec072c0 00009e10 00000000
	 [ 2.810000] 1da0: 9e549e18 9e549e10 ffffffed 805f81f4 fffffdfb 00000000 00000000 00000000
	 [ 2.810000] 1dc0: 9e4a1dec 9e4a1dd0 80243df8 802928ac 9e549e10 8062cbd8 8062cbe0 805f81f4
	 [ 2.810000] 1de0: 9e4a1e24 9e4a1df0 80242178 80243da4 803001d0 802ffa60 9e4a1e24 9e549e10
	 [ 2.810000] 1e00: 9e549e44 805f81f4 00000000 00000000 805b8840 8058a6b0 9e4a1e44 9e4a1e28
	 [ 2.810000] 1e20: 80242434 80241f04 00000000 805f81f4 80242344 00000000 9e4a1e6c 9e4a1e48
	 [ 2.810000] 1e40: 80240148 80242350 9e425bac 9e4fdc90 9e790e94 805f81f4 9e790e60 805f5640
	 [ 2.810000] 1e60: 9e4a1e7c 9e4a1e70 802425dc 802400d8 9e4a1ea4 9e4a1e80 80240ba8 802425c0
	 [ 2.810000] 1e80: 8050b6ac 9e4a1e90 805f81f4 ffffe000 805b8838 80616720 9e4a1ebc 9e4a1ea8
	 [ 2.810000] 1ea0: 80243068 80240a68 805ab24c ffffe000 9e4a1ecc 9e4a1ec0 80244a38 80242fec
	 [ 2.810000] 1ec0: 9e4a1edc 9e4a1ed0 805ab264 80244a04 9e4a1f4c 9e4a1ee0 8058ae70 805ab258
	 [ 2.810000] 1ee0: 80032c68 801e3fd8 8052f800 8041af2c 9e4a1f4c 9e4a1f00 80032f90 8058a6bc
	 [ 2.810000] 1f00: 9e4a1f2c 9e4a1f10 00000006 00000006 00000000 8052f220 805112f0 00000000
	 [ 2.810000] 1f20: 9e4a1f4c 00000006 80616720 805cf400 80616720 805b8838 80616720 00000057
	 [ 2.810000] 1f40: 9e4a1f94 9e4a1f50 8058b040 8058add0 00000006 00000006 00000000 8058a6b0
	 [ 2.810000] 1f60: 3940bf3d 00000007 f115c2e8 00000000 803fd158 00000000 00000000 00000000
	 [ 2.810000] 1f80: 00000000 00000000 9e4a1fac 9e4a1f98 803fd170 8058af38 00000000 803fd158
	 [ 2.810000] 1fa0: 00000000 9e4a1fb0 8000a5e8 803fd164 00000000 00000000 00000000 00000000
	 [ 2.810000] 1fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
	 [ 2.810000] 1fe0: 00000000 00000000 00000000 00000000 00000013 00000000 d11dcae8 af8ddec5
	 [ 2.810000] [<80331004>] (netif_napi_del) from [<80292b30>] (ftgmac100_probe+0x290/0x674)
	 [ 2.810000] [<80292b30>] (ftgmac100_probe) from [<80243df8>] (platform_drv_probe+0x60/0xc0)
	 [ 2.810000] [<80243df8>] (platform_drv_probe) from [<80242178>] (driver_probe_device+0x280/0x44c)
	 [ 2.810000] [<80242178>] (driver_probe_device) from [<80242434>] (__driver_attach+0xf0/0x104)
	 [ 2.810000] [<80242434>] (__driver_attach) from [<80240148>] (bus_for_each_dev+0x7c/0xb0)
	 [ 2.810000] [<80240148>] (bus_for_each_dev) from [<802425dc>] (driver_attach+0x28/0x30)
	 [ 2.810000] [<802425dc>] (driver_attach) from [<80240ba8>] (bus_add_driver+0x14c/0x268)
	 [ 2.810000] [<80240ba8>] (bus_add_driver) from [<80243068>] (driver_register+0x88/0x104)
	 [ 2.810000] [<80243068>] (driver_register) from [<80244a38>] (__platform_driver_register+0x40/0x54)
	 [ 2.810000] [<80244a38>] (__platform_driver_register) from [<805ab264>] (ftgmac100_driver_init+0x18/0x20)
	 [ 2.810000] [<805ab264>] (ftgmac100_driver_init) from [<8058ae70>] (do_one_initcall+0xac/0x168)
	 [ 2.810000] [<8058ae70>] (do_one_initcall) from [<8058b040>] (kernel_init_freeable+0x114/0x1cc)
	 [ 2.810000] [<8058b040>] (kernel_init_freeable) from [<803fd170>] (kernel_init+0x18/0x104)
	 [ 2.810000] [<803fd170>] (kernel_init) from [<8000a5e8>] (ret_from_fork+0x14/0x2c)
	 [ 2.810000] Code: e594205c e5941058 e2843058 e3a05000 (e5812004)
	 [ 3.210000] ---[ end trace f32811052fd3860c ]---

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>