drivers: usb: udc_dwc2: Notify upper layer on failed TX

Change ensures that `dwc2_handle_xfer_next` would notify upper layer if
`dwc2_tx_fifo_write` fails. This is necessary to ensure that upper layer
is aware of the failed TX for the submitted transfer. It also ensures
that the submitted transfer is removed from the TX queue.

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>
Signed-off-by: Kamil Piszczek <Kamil.Piszczek@nordicsemi.no>