coresight-tmc-etr: fix memory leak

When a new sys_buf needs to be allocated, the previously sys_buf
will not be freed. this will cause a memory leak. This change fix
this issue.

Change-Id: Ie5c2270b736271e5724cc83f76d9d9f4cd943088
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>