mac802154: move wpan.c to iface.c

The wpan.c file contains the interface handling functions now. It's similar
like the mac80211 iface.c file. This patch renames this file to iface.c to
have similar naming convention in mac802154.

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