acpi-cpufreq: remove unreliable device.get() code

acpi-cpufreq: remove unreliable device.get() code

cpufreq offers the optional driver.get() entry point
for drivers to export instantaneous frequency in
/sys/devices/system/cpu/cpu*/cpufreq/cpuinfo_cur_freq.

25% of the acpi-cpufreq driver is involved in supporting
that optional feature, but on modern processors, it
is not reliable.

So here we delete this optional feature from acpi-cpufreq.
/sys/devices/system/cpu/cpu*/cpufreq/cpuinfo_cur_freq
will go away on acpi-cpufreq systems, but note that
/sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq
will still be presnet to indicate the most recent request.

(and yes, powertop still works:-)

The most common reason that driver.get() is not reliable
is that modern processors autonomously change frequency
without OS instruction.  This means that reading
PERF_STATUS is possibly in-accurate as soon as the
instruction after it is read.

Average frequency over an interval is more useful
than instantaneous frequency on modern hardware.
acpi-cpufreq supplies average frequency via
the the driver->getavg() entry, which is what
the ondemand governor uses.

Signed-off-by: Len Brown <len.brown@intel.com>