Bluetooth: Refactor advertising report processing into its own function

As preparation for merging ADV_IND/ADV_SCAN_IND and SCAN_RSP together
into a single mgmt Device Found event refactor individual advertising
report handling into a separate function. This will help keep the code
more readable as more logic gets added.

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