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

PS2: Limit log spam (ALOGW -> ALOGV)

Signed-off-by: bmc08gt <bmc08gt.teamb00sted@gmail.com>
Change-Id: I2591028676cf9eca4978f1d8a88bae2ad5815109