net: openvswitch: return an error instead of doing BUG_ON()

For all other error cases in queue_userspace_packet() the error is
returned, so it makes sense to do the same for these two error cases.

Reported-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: David S. Miller <davem@davemloft.net>