net/mlx5: fix num seg assumption in SSE Tx

net/mlx5: fix num seg assumption in SSE Tx

vPMD Tx function assumes that after the scatter of the
multi-segment packets the next packet will be a single segment packet.

This is not current as the function can return due to lack of resources
without sending all of the multi-segment mbufs sequence.

Fixes: 6cb559d67b83 ("net/mlx5: add vectorized Rx/Tx burst for x86")
Cc: stable@dpdk.org
Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>