USB: Allow udc_probe to return success when registering trace fails

mdm9x15 currently does not support tracepoints so trying to
register a tracepoint would result in udc_probe failure.
This fix allows udc_probe to return success even if registering
the trace fails.

Change-Id: I8a616fc79de5c34c19fc9d19b66a0cdde76db990
Signed-off-by: Bar Weiner <bweiner@codeaurora.org>