net: sfp: Stop SFP polling and interrupt handling during shutdown

SFP device polling can cause problems during the shutdown process if the
parent devices of the network controller have been shut down already.
This problem was seen on the iMX6 platform with PCIe devices, where
accessing the device after the bus is shut down causes a hang.

Free any acquired GPIO interrupts and stop all delayed work in the SFP
driver during the shutdown process, so that we ensure that no pending
operations are still occurring after the SFP shutdown completes.

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