Bluetooth: btusb: Comment on unbalanced pm reference

Add a comment clarifying that a PM reference in btusb_qca_cmd_timeout is
not unbalanced because it results in a device reset.

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