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>