coresight: tmc: Fix use after free issue with tmc read

Fix race condition seen between reading tmc buffer and enabling
the device. The race condition can result in a use after free
issue if the buffer is released while a read is in progress.

Signed-off-by: Saranya Chidura <schidura@codeaurora.org>
Change-Id: I9908fa78acbf3152ee791c63fef525f09a9a23d5