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

Signed-off-by: bmc08gt <bmc08gt.teamb00sted@gmail.com>
Change-Id: I186b9bddd9518315e6f24bbeef1d131b6223996e