Bluetooth: Add hci_bdaddr_is_rpa convenience function

When implementing support for Resolvable Private Addresses (RPAs) we'll
need to in several places be able to identify such addresses. This patch
adds a simple convenience function to do the identification of the
address type.

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