usb: dwc2: hcd: fix build warning

commit db62b9a804b4 (usb: dwc2: host: don't
use dma_alloc_coherent with irqs disabled)
introduced a build warning by using NULL
as an integer. Fix that by just using 0 instead
of NULL.

Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/dwc2/hcd.c

@@ -749,7 +749,7 @@ static int dwc2_hc_setup_align_buf(struct dwc2_hsotg *hsotg, struct dwc2_qh *qh,
 			chan->ep_is_in ? DMA_FROM_DEVICE : DMA_TO_DEVICE);
 	if (dma_mapping_error(hsotg->dev, qh->dw_align_buf_dma)) {
 		dev_err(hsotg->dev, "can't map align_buf\n");
-		chan->align_buf = (dma_addr_t)NULL;
+		chan->align_buf = 0;
 		return -EINVAL;
 	}