net: phy: mscc: do not access the MDIO bus lock directly

This patch improves the MSCC driver by using the provided
phy_lock_mdio_bus and phy_unlock_mdio_bus helpers instead of locking and
unlocking the MDIO bus lock directly. The patch is only cosmetic but
should improve maintenance and consistency.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>