net: dsa: mv88e6xxx: Fix false positive lockdep splat

DSA can have nested MDIO busses, where the Ethernet MDIO bus is used
to access an MDIO bus within the switch which has the PHYs connected
to it. This nesting causes lockdep to give false positives. Use
mutex_lock_nested() to avoid this.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>