net/mlx5: Add support for VNIC_ENV internal rq counter

Add mlx5 interface support for reading internal rq out of buffer counter
as part of QUERY_VNIC_ENV command. The command is used by the driver to
query vnic diagnostic statistics from FW.

Signed-off-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>