net: mvmdio: remove duplicate locking

The MDIO layer already provides per-bus locking, so there's no need for
MDIO bus drivers to do their own internal locking.  Remove this.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>