mac802154: add wpan_phy priv id

This patch adds an unique id for an wpan_phy. This behaviour is mostly
grabbed from wireless stack. This is needed for upcomming patches which
identify the wpan netdev while NETDEV_CHANGENAME in netdev notify function.

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