net: sky2: Fix shutdown crash

The sky2 frequently crashes during machine shutdown with:

sky2_get_stats+0x60/0x3d8 [sky2]
dev_get_stats+0x68/0xd8
rtnl_fill_stats+0x54/0x140
rtnl_fill_ifinfo+0x46c/0xc68
rtmsg_ifinfo_build_skb+0x7c/0xf0
rtmsg_ifinfo.part.22+0x3c/0x70
rtmsg_ifinfo+0x50/0x5c
netdev_state_change+0x4c/0x58
linkwatch_do_dev+0x50/0x88
__linkwatch_run_queue+0x104/0x1a4
linkwatch_event+0x30/0x3c
process_one_work+0x140/0x3e0
worker_thread+0x60/0x44c
kthread+0xdc/0xf0
ret_from_fork+0x10/0x50

This is caused by the sky2 being called after it has been shutdown.
A previous thread about this can be found here:

https://lkml.org/lkml/2016/4/12/410



An alternative fix is to assure that IFF_UP gets cleared by
calling dev_close() during shutdown. This is similar to what the
bnx2/tg3/xgene and maybe others are doing to assure that the driver
isn't being called following _shutdown().

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>