net/mlx5e: XDP, Allow non-linear single-segment frames in XDP TX MPWQE

Under a few restrictions, TX MPWQE feature can serve multiple TX packets
in a single TX descriptor. It requires each of the packets to have a
single scatter entry / segment.

Today we allow only linear frames to use this feature, although there's
no real problem with non-linear ones where the whole packet reside in
the first fragment.

Expand the XDP TX MPWQE feature support to include such frames. This is
in preparation for the downstream patch, in which we will generate such
non-linear frames.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>