UPSTREAM: coresight: tmc-etr: Do not clean trace buffer

We zero out the entire trace buffer used for ETR before it is enabled,
for helping with debugging. With the addition of scatter-gather mode,
the buffer could be bigger and non-contiguous.

Get rid of this step; if someone wants to debug, they can always add it
as and when needed.

Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

(Upstream commit 0f728a7f9f8fed5dfd86a628d871d572cb91942c).

Bug: 140266694
Change-Id: I913a9bdf93f412fbbf0ceb2a25ad09dd7ce99677
Signed-off-by: Yabin Cui <yabinc@google.com>