Coresight: byte-cntr: Don't read byte-cntr when etr is diasbled

Tmc etr buffer will be read when 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: Ia7037362dd973895f3b095a2ffbbab83570fb57e
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>