net: aquantia: Implement pci shutdown callback

We should close link and all NIC operations during shutdown.
On some systems graceful reboot never closes NIC interface on its own,
but only indicates pci device shutdown. Without explicit handler, NIC
rx rings continued to transfer DMA data into prepared buffers while CPU
rebooted already. That caused memory corruptions on soft reboot.

Signed-off-by: Igor Russkikh <igor.russkikh@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>