drivers: udc_dwc2: Process SETUP stage when allowed

DWC2 programming guide mentions that SETUP can only be processed by
application after DOEPINTn.SETUP interrupt. Not respecting this
requirement makes setting Global OUT NAK hang while waiting for
GOUTNAKEFF interrupt until the host starts next control transfer.
Global OUT NAK is necessary if the application ever wants to properly
cancel any ongoing transfer.

Change the processing to comply with the programming guide.

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