usb: introduce a separate config option for DM USB device

Using CONFIG_DM_USB for this purpose prevents using DM_USB for host and not
for device.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
index f1ca619..f9d8465 100644
--- a/drivers/usb/dwc3/core.c
+++ b/drivers/usb/dwc3/core.c
@@ -789,8 +789,7 @@
 MODULE_LICENSE("GPL v2");
 MODULE_DESCRIPTION("DesignWare USB3 DRD Controller Driver");
 
-#if CONFIG_IS_ENABLED(DM_USB)
-
+#if CONFIG_IS_ENABLED(DM_USB_GADGET)
 int dwc3_init(struct dwc3 *dwc)
 {
 	int ret;