UPSTREAM: coresight: tmc-etr: Properly set AUX buffer head in snapshot mode

Unify amongst sink drivers how the AUX ring buffer head is communicated
to user space.  That way the same algorithm in user space can be used to
determine where the latest data is and how much of it to access.

Signed-off-by: Mathieu Poirier <mathieu.poirier@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>

(Upstream commit 3ecb03022a25b3f089b93a35f608ea7ee6a244aa).

Bug: 140266694
Change-Id: I90ce5258c04b5196a93fd9833eb15b32dff9060b
Signed-off-by: Yabin Cui <yabinc@google.com>