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>

drivers/ieee802154/ieee802154_uart_pipe.c

@@ -84,7 +84,7 @@ static uint8_t *upipe_rx(uint8_t *buf, size_t *off)
 
 		goto flush;
 out:
-		net_buf_unref(nbuf);
+		net_nbuf_unref(nbuf);
 flush:
 		upipe->rx = false;
 		upipe->rx_len = 0;