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.

Change-Id: Ie6cc9a89d4c8f0cf75e8c705a537679516aec99a
Signed-off-by: Satyajit Desai <sadesai@codeaurora.org>