net/mlx5: DR, Fix SQ/RQ in doorbell bitmask

QP doorbell size is 16 bits.
Fixing sw steering's QP doorbel bitmask, which had 20 bits.

Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>