net/mlx5e: Add missing release firmware call

Once driver finishes flashing the firmware image, it should release it.

Fixes: 9c8bca26

 ("mlx5: Move firmware flash implementation to devlink")
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Reviewed-by: Aya Levin <ayal@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>