net: Rework gianfar driver to use of_mdio infrastructure.

This patch simplifies the driver by making use of more common code.

Tested on Freescale MPC8349emitxgp eval board

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>