net/mlx5_core: Clear doorbell record inside mlx5_db_alloc()

Do it in one place instead of every where the function is invoked

Signed-off-by: Achiad Shochat <achiad@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>