net: qcom/emac: add shutdown function

The shutdown function halts all DMA and interrupts, so that all
operations are discontinued when the system shuts down, e.g. via
kexec or a forced reboot.

Tested-by: Tyler Baicar <tbaicar@codeaurora.org>
Signed-off-by: Timur Tabi <timur@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>