perf/x86: Improve debug output in check_hw_exists()

It might be of interest which perfctr msr failed.

Signed-off-by: Robert Richter <robert.richter@amd.com>
[ added hunk to avoid GCC warn ]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1340217996-2254-5-git-send-email-robert.richter@amd.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>