PowerHAL: Only write to SAMPLING_RATE_ONDEMAND when using Ondemand

Prevents E/CM PowerHAL(  559): Error opening /sys/devices/system/cpu/cpufreq/ondemand/sampling_rate: No such file or directory
Limit log spam (ALOGW -> ALOGV)

Originated from this commit: https://github.com/CyanogenMod/android_device_samsung_qcom-common/commit/1fc615982ba265d301ba8f6fd2184b9a9a3acafb