net: usb: asix: fill null-ptr-deref in asix_suspend

When asix_suspend() is called dev->driver_priv might not have been
assigned a value, so we need to check that it's not NULL.

Found by syzkaller.

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
Modules linked in:
CPU: 0 PID: 24 Comm: kworker/0:1 Not tainted 4.14.0-rc4-43422-geccacdd69a8c #400
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Workqueue: usb_hub_wq hub_event
task: ffff88006bb36300 task.stack: ffff88006bba8000
RIP: 0010:asix_suspend+0x76/0xc0 drivers/net/usb/asix_devices.c:629
RSP: 0018:ffff88006bbae718 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: ffff880061ba3b80 RCX: 1ffff1000c34d644
RDX: 0000000000000001 RSI: 0000000000000402 RDI: 0000000000000008
RBP: ffff88006bbae738 R08: 1ffff1000d775cad R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8800630a8b40
R13: 0000000000000000 R14: 0000000000000402 R15: ffff880061ba3b80
FS:  0000000000000000(0000) GS:ffff88006c600000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ff33cf89000 CR3: 0000000061c0a000 CR4: 00000000000006f0
Call Trace:
 usb_suspend_interface drivers/usb/core/driver.c:1209
 usb_suspend_both+0x27f/0x7e0 drivers/usb/core/driver.c:1314
 usb_runtime_suspend+0x41/0x120 drivers/usb/core/driver.c:1852
 __rpm_callback+0x339/0xb60 drivers/base/power/runtime.c:334
 rpm_callback+0x106/0x220 drivers/base/power/runtime.c:461
 rpm_suspend+0x465/0x1980 drivers/base/power/runtime.c:596
 __pm_runtime_suspend+0x11e/0x230 drivers/base/power/runtime.c:1009
 pm_runtime_put_sync_autosuspend ./include/linux/pm_runtime.h:251
 usb_new_device+0xa37/0x1020 drivers/usb/core/hub.c:2487
 hub_port_connect drivers/usb/core/hub.c:4903
 hub_port_connect_change drivers/usb/core/hub.c:5009
 port_event drivers/usb/core/hub.c:5115
 hub_event+0x194d/0x3740 drivers/usb/core/hub.c:5195
 process_one_work+0xc7f/0x1db0 kernel/workqueue.c:2119
 worker_thread+0x221/0x1850 kernel/workqueue.c:2253
 kthread+0x3a1/0x470 kernel/kthread.c:231
 ret_from_fork+0x2a/0x40 arch/x86/entry/entry_64.S:431
Code: 8d 7c 24 20 48 89 fa 48 c1 ea 03 80 3c 02 00 75 5b 48 b8 00 00
00 00 00 fc ff df 4d 8b 6c 24 20 49 8d 7d 08 48 89 fa 48 c1 ea 03 <80>
3c 02 00 75 34 4d 8b 6d 08 4d 85 ed 74 0b e8 26 2b 51 fd 4c
RIP: asix_suspend+0x76/0xc0 RSP: ffff88006bbae718
---[ end trace dfc4f5649284342c ]---

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>