smsc75xx: fix error handling in suspend failure case

This patch ensures that if we fail to suspend the LAN7500 device
we call usbnet_resume before returning failure, instead of
leaving the usbnet driver in an unusable state.

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