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>

drivers/usb/dwc3/Makefile

@@ -3,7 +3,11 @@ CFLAGS_trace.o				:= -I$(src)
 
 obj-$(CONFIG_USB_DWC3)			+= dwc3.o
 
-dwc3-y					:= core.o debug.o trace.o
+dwc3-y					:= core.o
+
+ifneq ($(CONFIG_FTRACE),)
+	dwc3-y				+= debug.o trace.o
+endif
 
 ifneq ($(filter y,$(CONFIG_USB_DWC3_HOST) $(CONFIG_USB_DWC3_DUAL_ROLE)),)
 	dwc3-y				+= host.o

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 *);