Bluetooth: Fix trying to disable scanning twice

The discovery process has a timer for disabling scanning, however
scanning might be disabled through other means too like the auto-connect
process.  We should therefore ensure that the timer is never active
after sending a HCI command to disable scanning.

There was some existing code in stop_scan_complete trying to avoid the
timer when a connect request interrupts a discovery procedure, but the
other way around was not covered. This patch covers both scenarios by
canceling the timer as soon as we get a successful command complete for
the disabling HCI command.

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