coresight-tmc-etr: Fix conditional check in TMC read

The current check will cause all of the ETR read
transactions to fail. Fix it so that we only
fail the read if byte-cntr is active.

Change-Id: I0d6ef03e692bebc8b775a399633b28c5a3158b0d
Signed-off-by: Satyajit Desai <sadesai@codeaurora.org>