Bluetooth: hci_persistent_key should return bool

This patch changes the return type of function hci_persistent_key
from int to bool because it makes more sense to return information
whether a key is persistent or not as a bool.

Signed-off-by: Vishal Agarwal <vishal.agarwal@stericsson.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>