Bluetooth: Add clarifying comment to command status handling

When dealing with HCI command status events, the reasoning for trying to
mark a request as complete if no specific event is being waited for and
status was success is not self-evident. This patch adds a clarifying
comment above the if-statement.

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