net/fsl: fix a bug in xgmac_mdio

There is a bug in xgmac_mdio_read when clear the bit MDIO_STAT_ENC,
which '&' is missed in 'mdio_stat &= ~MDIO_STAT_ENC'.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>