net/mlx5: Remove spinlock support from mlx5_write64

As there is no user of mlx5_write64 that passes a spinlock to
mlx5_write64, remove this functionality and simplify the function.

Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>

drivers/infiniband/hw/mlx5/qp.c

@@ -5015,7 +5015,7 @@ out:
 		wmb();
 
 		/* currently we support only regular doorbells */
-		mlx5_write64((__be32 *)ctrl, bf->bfreg->map + bf->offset, NULL);
+		mlx5_write64((__be32 *)ctrl, bf->bfreg->map + bf->offset);
 		/* Make sure doorbells don't leak out of SQ spinlock
 		 * and reach the HCA out of order.
 		 */