wireless network devices: Attach radio to d_buf before registering device to handle forwarding case.
When CONFIG_NET_IPFORWARD is enabled, and CONFIG_NET_6LOWPAN is being used, a packet that attempts to get forwarded on the 6LoWPAN interface will require that the radio's buffer be attached to d_buf. Otherwise the below exception will be hit.
~line 542 of sixlowpan_framelist.c
```
/* Recover the reassembly buffer from the driver d_buf. */
reass = (FAR struct sixlowpan_reassbuf_s *)radio->r_dev.d_buf;
DEBUGASSERT(reass != NULL);
```
The underlying "radio" in this case is the mac802154_netdev.
This behavior has probably not been observed because the buffer is normally attached in the periodic txpoll worker. However, in my case, the 6LoWPAN interace was not UP yet, and therefore the worker hadn't run yet.
Approved-by: Gregory Nutt <gnutt@nuttx.org>
535db0140a