coresight: tmc-etr: Rearrange probing default buffer size

As we are about to refactor the platform specific handling,
make the default buffer size probing generic.

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>