drivers: udc_dwc2: Rework control endpoint feeding

In Completer mode SETUP data can always be received and therefore
endpoint 0 should only be enabled for OUT Data Stage and OUT Status
Stage.

In Buffer DMA mode, SETUP can only be received when endpoint is enabled
and therefore the software has to make sure that there is a buffer
available to receive SETUP data.

Rework the EP0 buffer feeding to adhere to DWC2 Programming Guide.
Synchronize the accesses with driver mutex to avoid interrupt related
race conditions.

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