net: bcmgenet: use dev->phydev instead of priv->phydev

Now that the software reset of the PHY has been removed it is no
longer necessary to retain a private pointer to the phydev for
use when the PHY is detached (which isn't generally safe anyway).

The driver now uses the phydev member attached to the net_device.

For ethtool commands that have a PHY component, an explicit check
is made to prevent accessing an invalid phydev pointer when one
is not attached (e.g. interface is down).

Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>