ieee802154: fakelb: add deprecated msg while probe

Since mac802154_hwsim the fakelb driver will get deprecated. This patch will
notifier all users of fakelb to switch to the new mac802154_hwsim driver.

Signed-off-by: Alexander Aring <aring@mojatatu.com>
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>