Bluetooth: Use extra variable to make code more readable

When starting active scanning for discovery the whitelist is not needed
to be used. So the filter_policy is 0x00. To make the core more readable
use a variable name instead of just setting 0 as paramter.

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