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/drivers/usb/gadget/g_dnl.c b/drivers/usb/gadget/g_dnl.c
index 29d08a3..40868c0 100644
--- a/drivers/usb/gadget/g_dnl.c
+++ b/drivers/usb/gadget/g_dnl.c
@@ -15,7 +15,7 @@
 
 #include <g_dnl.h>
 #include <usb_mass_storage.h>
-#include "f_dfu.h"
+#include <dfu.h>
 
 #include "gadget_chips.h"
 #include "composite.c"