net: axienet: stop interface during shutdown

On some platforms, such as iMX6 with PCIe devices, crashes or hangs can
occur if the axienet device continues to perform DMA transfers after
parent devices/busses have been shut down. Shut down the axienet
interface during its shutdown callback in order to avoid this.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>