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-fsl.c b/drivers/usb/host/xhci-fsl.c
index 047a8df..c0b98a8 100644
--- a/drivers/usb/host/xhci-fsl.c
+++ b/drivers/usb/host/xhci-fsl.c
@@ -19,7 +19,7 @@
 #include <dm.h>
 
 /* Declare global data pointer */
-#ifndef CONFIG_DM_USB
+#if !CONFIG_IS_ENABLED(DM_USB)
 static struct fsl_xhci fsl_xhci;
 unsigned long ctr_addr[] = FSL_USB_XHCI_ADDR;
 #else
@@ -107,7 +107,7 @@
 	return 0;
 }
 
-#ifdef CONFIG_DM_USB
+#if CONFIG_IS_ENABLED(DM_USB)
 static int xhci_fsl_probe(struct udevice *dev)
 {
 	struct xhci_fsl_priv *priv = dev_get_priv(dev);