net: dsa: mv88e6xxx: Add missing mutex's in EEE operations.

The phy_mutex should be held while reading and writing to the phy.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>