usb: dwc3: fix sparse errors

sparse caught three mistakes on this driver,
fix them:

drivers/usb/dwc3/ep0.c:806:29: warning: duplicate const
drivers/usb/dwc3/debugfs.c:481:15: warning: symbol 'dwc3_debugfs_init' \
	was not declared. Should it be static?
drivers/usb/dwc3/debugfs.c:518:16: warning: symbol 'dwc3_debugfs_exit' \
	was not declared. Should it be static?

Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/drivers/usb/dwc3/debugfs.c b/drivers/usb/dwc3/debugfs.c
index e78abb4..87d403d 100644
--- a/drivers/usb/dwc3/debugfs.c
+++ b/drivers/usb/dwc3/debugfs.c
@@ -49,6 +49,7 @@
 #include "core.h"
 #include "gadget.h"
 #include "io.h"
+#include "debug.h"
 
 struct dwc3_register {
 	const char	*name;