ieee802154: handle socket functionality as module

This patch makes the ieee802154 socket handling as module. Currently
this is part of ieee802154 module. It pointed out that ieee802154 module
has also two module_init/module_exit functions. One inside of core.c and
the other in af_ieee802154.c. This patch will also solve this issue by
handle the af_802154 as separate module.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>