net/mlx5: fix asserts for multi-segment Tx offload misconfig

net/mlx5: fix asserts for multi-segment Tx offload misconfig

To support multi-segment packet sending the Tx datapath
DEV_TX_OFFLOAD_MULTI_SEGS offload must be configured.
In debug configuration the asserts arise if the multi-segment
packet is being sent and no DEV_TX_OFFLOAD_MULTI_SEGS is set.
It was found that in some execution paths asserts were missed,
this patch adds ones.

Fixes: 18a1c20044c0 ("net/mlx5: implement Tx burst template")
Cc: stable@dpdk.org
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>