Bluetooth: Add suspend reason for device disconnect

Update device disconnect event with reason 0x5 to indicate that device
disconnected because the controller is suspending.

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