usb:g_dnl:dfu: Download gadget and DFU function code clean up

The download gadget code and DFU function lacks of proper declarations
for the case when a target board wants to use only one of available usb
functions.

Moreover the relevant declarations have been moved to consistent
localization (like <dfu.h>).

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Cc: Marek Vasut <marex@denx.de>
diff --git a/include/dfu.h b/include/dfu.h
index 6f4bba4..b2ecf1b 100644
--- a/include/dfu.h
+++ b/include/dfu.h
@@ -14,6 +14,7 @@
 #include <common.h>
 #include <linux/list.h>
 #include <mmc.h>
+#include <linux/usb/composite.h>
 
 enum dfu_device_type {
 	DFU_DEV_MMC = 1,
@@ -161,4 +162,12 @@
 }
 #endif
 
+#ifdef CONFIG_DFU_FUNCTION
+int dfu_add(struct usb_configuration *c);
+#else
+int dfu_add(struct usb_configuration *c)
+{
+	return 0;
+}
+#endif
 #endif /* __DFU_ENTITY_H_ */