usb: mtu3: fix up undefined reference to usb_debug_root

When CONFIG_USB is not set, and CONFIG_USB_GADGET is set,
there is an issue:

ld:
drivers/usb/mtu3/mtu3_debugfs.o: in function 'ssusb_debugfs_create_root':
mtu3_debugfs.c:(.text+0xba3): undefined reference to 'usb_debug_root'

usb_debug_root is only built when CONFIG_USB is enabled, so here drop it
and use NULL instead.
Reported-by: NRandy Dunlap <rdunlap@infradead.org>
Signed-off-by: NChunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/mtu3/mtu3_debugfs.c

@@ -528,8 +528,7 @@ void ssusb_dr_debugfs_init(struct ssusb_mtk *ssusb)
 
 void ssusb_debugfs_create_root(struct ssusb_mtk *ssusb)
 {
-	ssusb->dbgfs_root =
-		debugfs_create_dir(dev_name(ssusb->dev), usb_debug_root);
+	ssusb->dbgfs_root = debugfs_create_dir(dev_name(ssusb->dev), NULL);
 }
 
 void ssusb_debugfs_remove_root(struct ssusb_mtk *ssusb)