Bluetooth: Clear discovery filter before starting background scan

Currently the discovery filter information are only cleared when the
actual discovery procedure has been stopped. To make sure that none
of the filters interfere with the background scanning and its device
found event reporting, clear the filter before starting background
scanning.

This means that the discovery filter is now cleared before either
Start Discovery, Start Service Discovery or background scanning.

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