prcmu-qos-power: update orig_min_freq only when policy->min was explicitly changed

Assumption that only sysfs can be the source of min/max freq changes was
wrong - u8500 hotplug driver and usecase gov can also do it...

Problematic situation occured when the phone went into sleep while
ARM_KHZ prcmu requirement was active:
- wake up the phone
- cause some wifi/mmc load to trigger ARM_KHZ qos (like dd if=/dev/block/mmc...
- turn screen off (this triggers the hotplug driver to set max freq
  limit which also permanently stored min freq increased by ARM_KHZ qos)
- kill the load (this resets the ARM_KHZ qos)
- wake up the phone -> min_freq is set to previos ARM_KHZ req value
  despite ARM_KHZ qos not being active any more