net/mlx5: fix Tx queue doorbell record field offset

net/mlx5: fix Tx queue doorbell record field offset

If the Send Queue (backing one for PMD Tx queue) the was
created with DevX API the doorbell record offset for the
producer index field was incorrect. If hardware missed the
doorbell register write event the wrong content of doorbell
record might cause queue malfunction. For the Send Queues
created with Verbs API the doorbell record offset was
configured correctly.

Fixes: 86d259cec852 ("net/mlx5: separate Tx queue object creations")
Cc: stable@dpdk.org
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>