drivers/ieee802154/pipe: Use net_nbuf_unref to release net_buf

Using net_nbuf_unref to release the net_buf so that we can
debug the allocations more easily. It is ok to use the original
net_buf_unref() too, we just miss some important net_buf
housekeeping information if done like that.

Change-Id: Ieb7b39ed525bfc46eb5c07a01f2a3a75fdbeb9fd
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>