coresight: tmc: Fix buffer size can't change issue

The sysfs interface "buffer_size" is not workable due to drvdata->etr_buf
is valid. When the buffer size changed by userspace, the memory did not
re-allocated. To fix this free allocated memory when ETR sink is disabled.

Change-Id: I3a8b85408a60c4f54a2ff5b6ac83236ed1c985c5
Signed-off-by: Mulu He <muluhe@codeaurora.org>