net: 6lowmac: Release net_buf if the packet is discarded

Important to release the net_buf if we discard the packet.
In the future it might be easier if input() would return
status and caller could discard the net_buf if needed.

Change-Id: I533a37ef7f2e80d99ad62301562228189a5e0308
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>