net/mlx5_core: fix an error code

We return success if mlx5e_alloc_sq_db() fails but we should return an
error code.

Fixes: f62b8bb8 ('net/mlx5: Extend mlx5_core to support ConnectX-4 Ethernet functionality')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>