net/mlx5: Split mac address setting function for using state_lock

Refactor mac address setting function to let caller hold the necessary
state_lock mutex, so that subsequent patch and use this helper routine.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>