vitio::net: Fix the description of try_xmit_one_locked(net_req*)

try_xmit_one_locked(net_req*) doesn't return EINVAL anymore. The appropriate
code has moved to xmit_prep().

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>