ieee802154: 6lowpan: Drop PACKET_OTHERHOST skbs in 6lowpan

There is no point processing pkts which are PACKET_OTHERHOST
in 6lowpan as they are discarded as soon as they reach the
ipv6 layer. Therefore we should drop them in the 6lowpan layer.

Signed-off-by: Simon Vincent <simon.vincent@xsilon.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>