net: phy: mscc: remove unnecessary phydev locking

Holding the struct phy_device (phydev) lock is unnecessary when
accessing phydev->interface in the PHY driver .config_init method,
which is the only place that vsc85xx_rgmii_set_skews() is called from.

The phy_modify_paged() function implements required MDIO bus level
locking, which can not be achieved by a phydev lock.

Signed-off-by: David Epping <david.epping@missinglinkelectronics.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>