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: I44e0f8b95ddfae5acac6fcc59e78a7741aaa97cc
Signed-off-by: Satyajit Desai <sadesai@codeaurora.org>