smsc95xx: enable power saving mode during system suspend

This patch enables the device to enter its lowest power SUSPEND2
state during system suspend, instead of staying up using full power.

Patch updated to not add two pointers to .suspend & .resume.

Patch updated to replace BUG_ON with WARN_ON_ONCE and return.

Signed-off-by: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: David S. Miller <davem@davemloft.net>