net: fill in MODULE_DESCRIPTION()s for ieee802154

W=1 builds now warn if module is built without a MODULE_DESCRIPTION().
Add descriptions to ieee802154 modules.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Stefan Schmidt <stefan@datenfreihafen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>