ieee802154: rework wpan_phy index assignment

This patch reworks the wpan_phy index incrementation. It's now similar
like wireless wiphy index incrementation. We move the wpan_phy index
attribute inside of cfg802154_registered_device and use atomic
operations instead locking mechanism via wpan_phy_mutex.

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