net: fec: Clear and enable MIB counters on imx51

Both the IMX51 and IMX53 datasheet indicates that the MIB counters
should be cleared during setup. Otherwise random numbers are returned
via ethtool -S.  Add a quirk and a function to do this.

Tested on an IMX51.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>