usb: s/CONFIG_DM_USB/CONFIG_IS_ENABLED(DM_USB)/

This allows to disable the USB driver model in SPL because it checks
the CONFIG_SPL_DM_USB variable for SPL builds. Nothing changes for
regular non-SPL builds.

Signed-off-by: Sven Schwermer <sven@svenschwermer.de>
diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
index da5dbd9..04ab540 100644
--- a/drivers/usb/host/xhci-mem.c
+++ b/drivers/usb/host/xhci-mem.c
@@ -723,7 +723,7 @@
 	int slot_id = udev->slot_id;
 	int speed = udev->speed;
 	int route = 0;
-#ifdef CONFIG_DM_USB
+#if CONFIG_IS_ENABLED(DM_USB)
 	struct usb_device *dev = udev;
 	struct usb_hub_device *hub;
 #endif
@@ -739,7 +739,7 @@
 	/* Only the control endpoint is valid - one endpoint context */
 	slot_ctx->dev_info |= cpu_to_le32(LAST_CTX(1));
 
-#ifdef CONFIG_DM_USB
+#if CONFIG_IS_ENABLED(DM_USB)
 	/* Calculate the route string for this device */
 	port_num = dev->portnr;
 	while (!usb_hub_is_root_hub(dev->dev)) {
@@ -782,7 +782,7 @@
 		BUG();
 	}
 
-#ifdef CONFIG_DM_USB
+#if CONFIG_IS_ENABLED(DM_USB)
 	/* Set up TT fields to support FS/LS devices */
 	if (speed == USB_SPEED_LOW || speed == USB_SPEED_FULL) {
 		struct udevice *parent = udev->dev;