usb: dwc3: don't compile dwc3_trace() unless CONFIG_FTRACE=y

We don't need dwc3_trace() unless we're building a
kernel with CONFIG_FTRACE. This patch reduces
dwc3.ko text size a bit while also removing overhead
of dwc3_trace() calls.

   text    data     bss     dec     hex filename
  50796     581       0   51377    c8b1 drivers/usb/dwc3/dwc3.o
  43961     581       0   44542    adfe drivers/usb/dwc3/dwc3.o.patched

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
diff --git a/drivers/usb/dwc3/debug.h b/drivers/usb/dwc3/debug.h
index b287fc6..d93780e 100644
--- a/drivers/usb/dwc3/debug.h
+++ b/drivers/usb/dwc3/debug.h
@@ -344,7 +344,13 @@ static inline const char *dwc3_gadget_generic_cmd_status_string(int status)
 	}
 }
 
+
+#if IS_ENABLED(CONFIG_FTRACE)
 void dwc3_trace(void (*trace)(struct va_format *), const char *fmt, ...);
+#else
+static inline void dwc3_trace(void (*trace)(struct va_format *), const char *fmt, ...)
+{  }
+#endif
 
 #ifdef CONFIG_DEBUG_FS
 extern void dwc3_debugfs_init(struct dwc3 *);