gianfar: Check if phydev present on ethtool -A

This fixes a seg fault on 'ethtool -A' entry if the
interface is down.  Obviously we need to have the
phy device initialized / "connected" (see of_phy_connect())
to be able to advertise pause frame capabilities.

Fixes: 23402bdd
Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>