net: smsc911x: add power management functions

This adds a power management implementation for smsc911x.c which assumes
the chips remains powered during suspend. The device is put in its D1
power saving mode.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Acked-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>