Bluetooth: ATT: Fix canceling ATT timeout upon response

For some write requests, such as CCC, the code doesn't use an ATT
request context but we still need to clear the request timeout when
the response comes. Move the k_delayed_work_cancel() call to the right
place and add some debug logs that helped pinpoint this issue.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>