net: ieee802154: Check reassembly return value properly

If IEEE 802.15.4 reassembly function ieee802154_reassemble() returns
anything other than NET_CONTINUE, then drop that packet. Earlier
it only dropped the packet if NET_DROP was returned but the reassembly
might also return NET_OK. In that case the pkt is freed already and
pkt->frags pointer is NULL. This caused NULL pointer access in L2 when
packet was received.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>