coresight: tmc: Fix the unbalanced lock in tmc_read()

'commit 734aabed17090 ("coresight: tmc: Fix use after free issue
with tmc read")' adds lock in tmc_read() to fix race condition seen in
reading tmc buffer and enabling the device.But commit has unbalanced
lock. This patch fixes the lock.

Change-Id: Iaf3ecd83ef5af346725885ea2c84c4185f1a1c50
Signed-off-by: Saranya Chidura <schidura@codeaurora.org>