add reset for deferral counter

[cornevandyk]

The current script doesn't reset the deferral counter. This causes the behaviour that, if ever a new desiredOS is set in the XML file, the update will be forced immediately, since the deferral counter is already at the limit.

This PR resets the deferral counter to 0 as late as possible before finalising the update, so that any future updates will also allow deferrals.