drivers: net: phy: xgene: Fix mdio write

This patches fixes a typo in the argument to xgene_enet_wr_mdio_csr().

Signed-off-by: Quan Nguyen <qnguyen@apm.com>
Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>