coresight: etm: perf: Fix warning caused by etm_setup_aux failure

coresight: etm: perf: Fix warning caused by etm_setup_aux failure

When coresight_build_path() fails on all the cpus, etm_setup_aux
calls etm_free_aux() to free allocated event_data.
WARN_ON(cpumask_empty(mask) will be triggered since cpu mask is empty.
Check event_data->snk_config is not NULL first to avoid this
warning.

Fixes: f5200aa9831f38 ("coresight: perf: Refactor function free_event_data()")
Reviewed-by: Mike Leach <mike.leach@linaro.org>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Tingwei Zhang <tingwei@codeaurora.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20200928163513.70169-9-mathieu.poirier@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>