Bluetooth: Rename get_adv_instance_scan_rsp

This renames get_adv_instance_scan_rsp to adv_instance_is_scannable and
make it return a bool since it was not actually properly return the size
of the scan response as one could expect.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>