gianfar: Implement proper, per netdevice wakeup management

This patch implements wakeup management for the gianfar driver.

The driver should set wakeup enable if WOL is enabled, so that
phylib won't power off an attached PHY.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>