Bluetooth: Refactor discovery stopping into its own function

We'll need to reuse the same logic for stopping discovery also when
cleaning up HCI state when powering off. This patch refactors the code
out to its own function that can later (in a subsequent patch) be used
also for the power off case.

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