net/mlx5: fix setting flags to external Rx queue

The flow_drv_rxq_flags_set sets the Rx queue flags (Mark/Flag and Tunnel
Ptypes) according to the device flow.

It tries to get the RxQ control structure to update its ptype. However,
external RxQs don't have control structure to update and it may cause a
crash.

This patch add check whether this Queue is external.

Fixes: 311b17e669ab ("net/mlx5: support queue/RSS actions for external Rx queue")
Cc: stable@dpdk.org
Signed-off-by: Michael Baum <michaelba@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>