acpi-cpufreq: Do not let get_measured perf depend on internal variable

Take already available policy->cpuinfo.max_freq and get rid of acpi-cpufreq
specific max_freq variable.

This implies that P0 is always the highest frequency which should always
be true as ACPI spec says:
As a result, the zeroth entry describes the highest performance state

Signed-off-by: Thomas Renninger <trenn@suse.de>
Acked-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>