net/virtio: refactor Tx offload helper

net/virtio: refactor Tx offload helper

Purely cosmetic but it is rather odd to have an "offload" helper that
checks if it actually must do something.
We already have the same checks in most callers, so move this branch
in them.

Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Flavio Leitner <fbl@sysclose.org>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>