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/common/usb_kbd.c b/common/usb_kbd.c
index fdeb2ae..020f0d4 100644
--- a/common/usb_kbd.c
+++ b/common/usb_kbd.c
@@ -539,7 +539,7 @@
 	return 0;
 }
 
-#ifndef CONFIG_DM_USB
+#if !CONFIG_IS_ENABLED(DM_USB)
 /* Search for keyboard and register it if found. */
 int drv_usb_kbd_init(void)
 {
@@ -602,7 +602,7 @@
 
 #endif
 
-#ifdef CONFIG_DM_USB
+#if CONFIG_IS_ENABLED(DM_USB)
 
 static int usb_kbd_probe(struct udevice *dev)
 {