net/mlx5e: Verify flow_source cap before using it

When adding send to vport rule verify flow_source matching is
supported by checking the flow_source cap.

Fixes: d0444254 ("net/mlx5: E-Switch, set flow source for send to uplink rule")
Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>