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.h b/drivers/usb/host/xhci.h
index a7555b2..6017504 100644
--- a/drivers/usb/host/xhci.h
+++ b/drivers/usb/host/xhci.h
@@ -1211,7 +1211,7 @@
 #define XHCI_STS_CNR		(1 << 11)
 
 struct xhci_ctrl {
-#ifdef CONFIG_DM_USB
+#if CONFIG_IS_ENABLED(DM_USB)
 	struct udevice *dev;
 #endif
 	struct xhci_hccr *hccr;	/* R/O registers, not need for volatile */