smsc75xx: enable power saving mode during system suspend

This patch instructs the device to enter its lowest power SUSPEND2
state during system suspend.

This patch also explicitly wakes the device after resume, which
should address reports of the device not automatically coming
back after system suspend:

Patch updated to change BUG_ON to WARN_ON_ONCE.

http://code.google.com/p/chromium-os/issues/detail?id=31871

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