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>
Signed-off-by: Michael Bestas <mikeioannina@gmail.com>

Change-Id: I563942194ae95758b45ed9aa76f24f96b28ff4d4