oprofile: Fix p6 counter overflow check

Fix the counter overflow check for CPUs with counter width > 32

I had a similar change in a different patch that I didn't submit
and I didn't notice the problem earlier because it was always
tested together.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Robert Richter <robert.richter@amd.com>