drivers: net: xgene: fix: Oops in linkwatch_fire_event

[ 1065.801569] Internal error: Oops: 96000006 [#1] SMP
...
[ 1065.866655] Hardware name: AppliedMicro Mustang/Mustang, BIOS 1.1.0 Apr 22 2015
[ 1065.873937] Workqueue: events_power_efficient phy_state_machine
[ 1065.879837] task: fffffe01de105e80 ti: fffffe00bcf18000 task.ti: fffffe00bcf18000
[ 1065.887288] PC is at linkwatch_fire_event+0xac/0xc0
[ 1065.892141] LR is at linkwatch_fire_event+0xa0/0xc0
[ 1065.896995] pc : [<fffffe000060284c>] lr : [<fffffe0000602840>] pstate: 200001c5
[ 1065.904356] sp : fffffe00bcf1bd00
...
[ 1066.196813] Call Trace:
[ 1066.199248] [<fffffe000060284c>] linkwatch_fire_event+0xac/0xc0
[ 1066.205140] [<fffffe000061167c>] netif_carrier_off+0x54/0x64
[ 1066.210773] [<fffffe00004f1654>] phy_state_machine+0x120/0x3bc
[ 1066.216578] [<fffffe00000d8d10>] process_one_work+0x15c/0x3a8
[ 1066.222296] [<fffffe00000d9090>] worker_thread+0x134/0x470
[ 1066.227757] [<fffffe00000df014>] kthread+0xe0/0xf8
[ 1066.232525] Code: 97f65ee9 f9420660 d538d082 8b000042 (885f7c40)

The fix is to call phy_disconnect() from xgene_enet_mdio_remove,
which in turn call cancel_delayed_work_sync().

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>