tools/power turbostat: Enable non-privileged users to read sysfs counters

A group of counters called "sysfs" displays software
C-state request counts and resulting perceived C-state residency.

They are not built-in counters that turbostat knows about ahead of time,
rather they are discovered in sysfs when turbostat starts.

Thus, they are added dynamically, using the same interface
as user-added MSR counters.

When turbostat enters "no-msr" mode, such as when running as a
non-privileged user, it clears all added counters.

Updating that to clear only actual MSR added counters
allows regular users to see the sysfs counters.

[lenb: commit message]

Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>