perf stat: Quieten failed to read counter message

perf stat: Quieten failed to read counter message

Since 3b3eb0445 running perf stat on a system without
backend-stalled-cycles spits out ugly warnings by default.

Since that is quite common, make the message a debug message only.

We know anyways that the counter wasn't read by the normal <unsupported>
output.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/1441147966-14917-1-git-send-email-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>