net/fsl: fix a bug in xgmac_mdio

There is a bug in xgmac_wait_until_done() which mdio_stat should be used
instead of mdio_data when checking if busy bit is cleared.

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