net/mlx5: Adjust log_max_qp to be 18 at most

The cited commit limited log_max_qp to be 17 due to FW capabilities.
Recently, it turned out that there are old FW versions that supported
more than 17, so the cited commit caused a degradation.

Thus, set the maximum log_max_qp back to 18 as it was before the
cited commit.

Fixes: 7f839965 ("net/mlx5: Update log_max_qp value to be 17 at most")
Signed-off-by: Maher Sanalla <msanalla@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>