nfp: allow TSO packets with metadata prepended in NFDK path

Packets with metadata prepended can be correctly handled in
firmware when TSO is enabled, now remove the error path and
related comments. Since there's no existing firmware that
uses prepended metadata, no need to add compatibility check
here.

Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Reviewed-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>