oprofile/x86: fix crash when profiling more than 28 events

With multiplexing enabled oprofile crashs when profiling more than 28
events. This patch fixes this.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Robert Richter <robert.richter@amd.com>