coresight: etb10: Cleanup power management

We drop the power before we complete the probe successfully. We
are supposed to drop it only when we are successful. Also, probing
the etb_buffer_length happens with the power turned up. So we don't
need to do that again in the helper.

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>