mvpp2: fix panic on module removal

mvpp2 uses a delayed workqueue to gather traffic statistics.
On module removal the workqueue can be destroyed before calling
cancel_delayed_work_sync() on its works.
Fix it by moving the destroy_workqueue() call after mvpp2_port_remove().
Also remove an unneeded call to flush_workqueue()

    # rmmod mvpp2
    [ 2743.311722] mvpp2 f4000000.ethernet eth1: phy link down 10gbase-kr/10Gbps/Full
    [ 2743.320063] mvpp2 f4000000.ethernet eth1: Link is Down
    [ 2743.572263] mvpp2 f4000000.ethernet eth2: phy link down sgmii/1Gbps/Full
    [ 2743.580076] mvpp2 f4000000.ethernet eth2: Link is Down
    [ 2744.102169] mvpp2 f2000000.ethernet eth0: phy link down 10gbase-kr/10Gbps/Full
    [ 2744.110441] mvpp2 f2000000.ethernet eth0: Link is Down
    [ 2744.115614] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
    [ 2744.115615] Mem abort info:
    [ 2744.115616]   ESR = 0x96000005
    [ 2744.115617]   Exception class = DABT (current EL), IL = 32 bits
    [ 2744.115618]   SET = 0, FnV = 0
    [ 2744.115619]   EA = 0, S1PTW = 0
    [ 2744.115620] Data abort info:
    [ 2744.115621]   ISV = 0, ISS = 0x00000005
    [ 2744.115622]   CM = 0, WnR = 0
    [ 2744.115624] user pgtable: 4k pages, 39-bit VAs, pgdp=0000000422681000
    [ 2744.115626] [0000000000000000] pgd=0000000000000000, pud=0000000000000000
    [ 2744.115630] Internal error: Oops: 96000005 [#1] SMP
    [ 2744.115632] Modules linked in: mvpp2(-) algif_hash af_alg nls_iso8859_1 nls_cp437 vfat fat xhci_plat_hcd m25p80 spi_nor xhci_hcd mtd usbcore i2c_mv64xxx sfp usb_common marvell10g phy_generic spi_orion mdio_i2c i2c_core mvmdio phylink sbsa_gwdt ip_tables x_tables autofs4 [last unloaded: mvpp2]
    [ 2744.115654] CPU: 3 PID: 8357 Comm: kworker/3:2 Not tainted 5.3.0-rc2 #1
    [ 2744.115655] Hardware name: Marvell 8040 MACCHIATOBin Double-shot (DT)
    [ 2744.115665] Workqueue: events_power_efficient phylink_resolve [phylink]
    [ 2744.115669] pstate: a0000085 (NzCv daIf -PAN -UAO)
    [ 2744.115675] pc : __queue_work+0x9c/0x4d8
    [ 2744.115677] lr : __queue_work+0x170/0x4d8
    [ 2744.115678] sp : ffffff801001bd50
    [ 2744.115680] x29: ffffff801001bd50 x28: ffffffc422597600
    [ 2744.115684] x27: ffffff80109ae6f0 x26: ffffff80108e4018
    [ 2744.115688] x25: 0000000000000003 x24: 0000000000000004
    [ 2744.115691] x23: ffffff80109ae6e0 x22: 0000000000000017
    [ 2744.115694] x21: ffffffc42c030000 x20: ffffffc42209e8f8
    [ 2744.115697] x19: 0000000000000000 x18: 0000000000000000
    [ 2744.115699] x17: 0000000000000000 x16: 0000000000000000
    [ 2744.115701] x15: 0000000000000010 x14: ffffffffffffffff
    [ 2744.115702] x13: ffffff8090e2b95f x12: ffffff8010e2b967
    [ 2744.115704] x11: ffffff8010906000 x10: 0000000000000040
    [ 2744.115706] x9 : ffffff80109223b8 x8 : ffffff80109223b0
    [ 2744.115707] x7 : ffffffc42bc00068 x6 : 0000000000000000
    [ 2744.115709] x5 : ffffffc42bc00000 x4 : 0000000000000000
    [ 2744.115710] x3 : 0000000000000000 x2 : 0000000000000000
    [ 2744.115712] x1 : 0000000000000008 x0 : ffffffc42c030000
    [ 2744.115714] Call trace:
    [ 2744.115716]  __queue_work+0x9c/0x4d8
    [ 2744.115718]  delayed_work_timer_fn+0x28/0x38
    [ 2744.115722]  call_timer_fn+0x3c/0x180
    [ 2744.115723]  expire_timers+0x60/0x168
    [ 2744.115724]  run_timer_softirq+0xbc/0x1e8
    [ 2744.115727]  __do_softirq+0x128/0x320
    [ 2744.115731]  irq_exit+0xa4/0xc0
    [ 2744.115734]  __handle_domain_irq+0x70/0xc0
    [ 2744.115735]  gic_handle_irq+0x58/0xa8
    [ 2744.115737]  el1_irq+0xb8/0x140
    [ 2744.115738]  console_unlock+0x3a0/0x568
    [ 2744.115740]  vprintk_emit+0x200/0x2a0
    [ 2744.115744]  dev_vprintk_emit+0x1c8/0x1e4
    [ 2744.115747]  dev_printk_emit+0x6c/0x7c
    [ 2744.115751]  __netdev_printk+0x104/0x1d8
    [ 2744.115752]  netdev_printk+0x60/0x70
    [ 2744.115756]  phylink_resolve+0x38c/0x3c8 [phylink]
    [ 2744.115758]  process_one_work+0x1f8/0x448
    [ 2744.115760]  worker_thread+0x54/0x500
    [ 2744.115762]  kthread+0x12c/0x130
    [ 2744.115764]  ret_from_fork+0x10/0x1c
    [ 2744.115768] Code: aa1403e0 97fffbbe aa0003f5 b4000700 (f9400261)

Fixes: 118d6298 ("net: mvpp2: add ethtool GOP statistics")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Acked-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>