net/mlx5: fix doorbell release on Rx queue release

net/mlx5: fix doorbell release on Rx queue release

Function mlx5_rxq_release() calls mlx5_release_dbr() to release the
doorbell allocated for this Rx queue.
This call is relevant only for Rx queue objects created using
DevX API.

This patch adds the required check, to call mlx5_release_dbr()
only when relevant.
It also updates mlx5_release_dbr() to use the input offset correctly.

Fixes: dc9ceff73c99 ("net/mlx5: create advanced RxQ via DevX")
Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>