ieee802154: socket: change module name

This patch changes the module name of af_802154 to ieee802154_socket.
Just for keeping the name convention according to the 6LoWPAN module
ieee802154_6lowpan.

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