Bluetooth: Don't try to look up private addresses as Identity Address

Identity Addresses are either public or static random. When looking up
addresses based on the Identity Address it doesn't make sense to go
through the IRK list if we're given a private random address. This patch
fixes (or rather improves) the hci_find_irk_by_addr function to bail out
early if given a private random address.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>