coresight: byte-cntr: add sanity check while reading the data

Add sanity check to ensure valid  access of kernel buffer
allocated for etr, while reading the byte-cntr device data.

Change-Id: I586bd5e2485f7e74456985b94a6792fac9b26d8a
Signed-off-by: Saranya Chidura <schidura@codeaurora.org>