net: mvneta: ensure PM paths take the rtnl lock

The netdev core always ensures that the rtnl lock is held while calling
the ndo_open() and ndo_stop() methods. However, the suspend/resume paths
do not hold the rtnl lock. phylink will expect the rtnl lock to be held
when the MAC driver calls it, so we end up with kernel warnings. Take
the lock to ensure that these functions are called in a consistent
manner.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>