s390/qeth: pass proto to qeth_l3_get_cast_type()

qeth_l3_hard_start_xmit() already determined the skb's proto. Avoid
doing so a second time when it calls qeth_l3_get_cast_type().

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>