coresight: tmc: skip tmc read if mem allocaiton failed

Add NULL pointer check to ensure that we do copy to user only
when we were able to allocate memory for ETR.

Change-Id: I09b4bc37617811fd4acd86a7e4f5ef91630675df
Signed-off-by: Satyajit Desai <sadesai@codeaurora.org>