net/mlx5: Use mlx5_sf_start_function_id() helper instead of directly calling MLX5_CAP_GEN()

There is a helper called mlx5_sf_start_function_id() that
wraps up a query to get base SF function id. Use that instead of
calling MLX5_CAP_GEN() directly.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>