ieee802154: rename af_ieee802154.c to socket.c

This patch renames the "af_ieee802154.c" to "socket.c". This is just a
cleanup to have a short name for it which describes the implementationm
stuff more human understandable.

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