net: fec: correct the MDIO clock source

Since imx serials FEC/ENET MDIO clock source is internal ipg clock,
and "ahb" clock is defined as FEC/ENET bus clock, so the patch just
correct the fec driver MDIO clock source.

Signed-off-by: Fugang Duan <B38611@freescale.com>
Acked-by: Frank Li <frank.li@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>