drivers: udc_dwc2: Fix isochronous endpoint disable

The NAKSts bit may be set on isochronous OUT endpoints when incomplete
ISO OUT interrupt is raised. The code would then assume that endpoint is
already disabled and would not perform the endpoint disable procedure.
This in turn was essentially halting any transmission on the isochronous
endpoint, abruptly terminating the data stream.

Fix the issue by always following full endpoint disable procedure on
isochronous endpoints.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>