eth_get_gso_type() routine returns segmentation offload type based on
L3 protocol type. It calls g_assert_not_reached if L3 protocol is
unknown, making the following return statement unreachable. Remove the
g_assert call, it maybe triggered by a guest user.

Reported-by: Gaoning Pan <pgn@zju.edu.cn>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>