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

(Upstream commit 3ecb03022a25b3f089b93a35f608ea7ee6a244aa).

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>

Bug: 140266694
Change-Id: I2a76f700070b5748493157c895989bddb14eb2ad
Signed-off-by: Yabin Cui <yabinc@google.com>