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_storage.c b/common/usb_storage.c
index 560d605..c9a99b1 100644
--- a/common/usb_storage.c
+++ b/common/usb_storage.c
@@ -299,7 +299,7 @@
 	if (mode == 1)
 		printf("       scanning usb for storage devices... ");
 
-#ifndef CONFIG_DM_USB
+#if !CONFIG_IS_ENABLED(DM_USB)
 	unsigned char i;
 
 	usb_disable_asynch(1); /* asynch transfer not allowed */
@@ -942,7 +942,7 @@
 	size_t __maybe_unused size;
 	int __maybe_unused ret;
 
-#ifndef CONFIG_DM_USB
+#if !CONFIG_IS_ENABLED(DM_USB)
 #ifdef CONFIG_USB_EHCI_HCD
 	/*
 	 * The U-Boot EHCI driver can handle any transfer length as long as
@@ -1495,7 +1495,7 @@
 	return 1;
 }
 
-#ifdef CONFIG_DM_USB
+#if CONFIG_IS_ENABLED(DM_USB)
 
 static int usb_mass_storage_probe(struct udevice *dev)
 {