smsc75xx: fix resume after device reset

On some systems this device fails to properly resume after suspend,
this patch fixes it by running the usbnet_resume handler.

I suspect this also fixes this bug:

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>