usb: dwc3: Makefile: fix link error on randconfig

If building a kernel without FTRACE but with TRACING, dwc3.ko fails to
link due to missing trace events. Fix this by using the correct
Kconfig symbol on Makefile.
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/Makefile

@@ -6,7 +6,7 @@ obj-$(CONFIG_USB_DWC3)			+= dwc3.o
 
 dwc3-y					:= core.o
 
-ifneq ($(CONFIG_FTRACE),)
+ifneq ($(CONFIG_TRACING),)
 	dwc3-y				+= trace.o
 endif