sched: Fix sched_cpu_legacy_freq() implementation

arch_scale_freq_capacity() returns the current CPU capacity scaled
to the maximum possible frequency (policy->cpuinfo.max_freq) but not
maximum frequency (policy->max) as per the current cpufreq policy. So
use the maximum possible frequency while deriving the current frequency
in sched_cpu_legacy_freq().

Change-Id: If4fd1ec26c5beaf1982f61c8be37dc064c8b7828
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>