coresight: tmc-etf: Fix snapshot mode update function

When working in snapshot mode function perf_aux_output_begin()
does not set the handle->size because the size is expected to be
deduced by the placement of the "head" and "old" pointers in user
space.  As such there is no point in trying to adjust the amount
of data to copy to the ring buffer.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Leo Yan <leo.yan@linaro.org>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Tested-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>