Bluetooth: Add hci_copy_identity_address convenience function

The number of places needing the local Identity Address are starting to
grow so it's better to have a single place for the logic of determining
it. This patch adds a convenience function for getting the Identity
Address and updates the two current places needing this to use it.

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