coresight: byte-cntr: stop reading byte-cntr when etr is diasbled

Tmc etr buffer will be read when user runs 'cat /dev/byte-cntr'.
If tmc etr is disabled, the buffer will be released. So exit
byte-cntr-read when etr is disabled.

Change-Id: Ie1a5367a76eec11563e4a5ff36f616ae40b620f2
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>