[CPUFREQ] cpufreq:userspace: fix cpu_cur_freq updation

CPU frequency is guranteed to be changed on notifier callback with
CPUFREQ_POSTCHANGE. Notifier callback with CPUFREQ_PRECHANGE does
not gurantee a change in frequency; after it, if cpufreq driver is
unable to change CPU to new frequency. This results in wrong
information being fed to user (if setting CPU frequency fails)
upon doing like,

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_setspeed

Hence in userspace governer update cpu_cur_freq only if notifier
has been called with POSTCHANGE.

Signed-off-by: Afzal Mohammed <afzal@ti.com>
Signed-off-by: Dave Jones <davej@redhat.com>