gianfar: Fix Wake-on-LAN support

commit 0f0ca340 ("phy: power
management support") caused a regression in the gianfar driver.

Now phylib turns off PHY power during suspend, and thus WOL
doesn't work anymore.

This patch workarounds the issue by enabling wakeup in the MDIO
device, i.e. just restores the old behaviour for the gianfar
driver. Note that this way all PHYs on a given MDIO bus won't
be turned off during suspend, which isn't good from the power
saving point of view.

A proper, per netdevice wakeup management support will need
a bit reworked phylib suspend/resume logic.

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