net/mlx5e: Use mlx5e_stop_room_for_max_wqe where appropriate

mlx5e_alloc_xdpsq calculates sq->stop_room internally, but there is
already a function for that: mlx5e_stop_room_for_max_wqe. This commit
makes use of this function.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>