Bluetooth: Simplify the code for re-arming discoverable timeout

When only the discoverable timeout gets updated, just cancel the current
timeout, store the new timeout value. If the new timeout is valid, then
arm the discoverable timeout again.

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