Bluetooth: Add a define for the HCI persistent flags mask

We'll need to use this mask also when powering off the HCI device
so it's better to have this in a single and visible place.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>