Bluetooth: Clear suspend tasks on unregister

While unregistering, make sure to clear the suspend tasks before
cancelling the work. If the unregister is called during resume from
suspend, this will unnecessarily add 2s to the resume time otherwise.

Fixes: 4e8c36c3

 (Bluetooth: Fix suspend notifier race)
Signed-off-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>