USB: host: fix sparse warning: Using plain integer as NULL pointer

Fix this sparse warning:
 drivers/usb/host/oxu210hp-hcd.c:2687:42: warning: Using plain integer as NULL pointer
Signed-off-by: NHannes Eder <hannes@hanneseder.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/host/oxu210hp-hcd.c

@@ -2684,7 +2684,7 @@ static int oxu_reset(struct usb_hcd *hcd)
 	oxu->urb_len = 0;
 
 	/* FIMXE */
-	hcd->self.controller->dma_mask = 0UL;
+	hcd->self.controller->dma_mask = NULL;
 
 	if (oxu->is_otg) {
 		oxu->caps = hcd->regs + OXU_OTG_CAP_OFFSET;