ieee802154: 6lowpan: rename internal header

This patch renames the internal header for af802154. This naming
convention is like ieee802154_i.h in mac802154 and avoids naming
confusing with the global af802154 header. Furthermore this header
contains more ieee802154 specific definitions.

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