mac802154: use IEEE802154_EXTENDED_ADDR_LEN

This patch removes the af_ieee802154 defines and use the
IEEE802154_EXTENDED_ADDR_LEN. We should do this everywhere in the
802.15.4 subsystem because af_ieee802154 should be normally an uapi
header.

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