Bluetooth: Cancel le_scan_restart work when stopping discovery

Not cancelling it has caused a bug where passive background scanning is
disabled out of the blue, preventing BLE keyboards/mice to reconnect.
Here is how it happens:
After hci_req_stop_discovery, there is still le_scan_restart_work
scheduled. Invocation of le_scan_restart_work causes a harmful
le_scan_disable_work to be scheduled. This le_scan_disable_work will
eventually disable passive scanning when the timer fires.

Sample btmon trace:

< HCI Command: LE Set Scan Parameters (0x08|0x000b) plen 7
        Type: Passive (0x00)
        Interval: 367.500 msec (0x024c)
        Window: 37.500 msec (0x003c)
        Own address type: Public (0x00)
        Filter policy: Accept all advertisement (0x00)
> HCI Event: Command Complete (0x0e) plen 4
      LE Set Scan Parameters (0x08|0x000b) ncmd 1
        Status: Success (0x00)
< HCI Command: LE Set Scan Enable (0x08|0x000c) plen 2
        Scanning: Enabled (0x01)
        Filter duplicates: Disabled (0x00)
> HCI Event: Command Complete (0x0e) plen 4
      LE Set Scan Enable (0x08|0x000c) ncmd 2
        Status: Success (0x00)
...
< HCI Command: LE Set Scan Enable (0x08|0x000c) plen 2
        Scanning: Disabled (0x00)
        Filter duplicates: Disabled (0x00)
> HCI Event: Command Complete (0x0e) plen 4
      LE Set Scan Enable (0x08|0x000c) ncmd 2
        Status: Success (0x00)
// Background scanning is not working here onwards.

Reviewed-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Signed-off-by: Sonny Sasaka <sonnysasaka@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>