Bluetooth: Create hci_req_add_le_passive_scan helper

This patches creates the public hci_req_add_le_passive_scan helper so
it can be re-used outside hci_core.c in the next patch.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>