Bluetooth: hci_conn: Use disable_delayed_work_sync

This makes use of disable_delayed_work_sync instead
cancel_delayed_work_sync as it not only cancel the ongoing work but also
disables new submit which is disarable since the object holding the work
is about to be freed.

Reported-by:  <syzbot+2446dd3cb07277388db6@syzkaller.appspotmail.com>
Tested-by:  <syzbot+2446dd3cb07277388db6@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=2446dd3cb07277388db6


Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>