Currently if ENABLE_VS is off, each time we poll TMP006 sensors, it
takes 1 second for each sensor to return ERROR. Checking for power
before polling help to reduce the time spent on temperature polling.
Signed-off-by: Vic Yang <victoryang@chromium.org>
BUG=chrome-os-partner:8275
TEST=Manual test
Change-Id: I2da83f09cccc331074f9bb5483b2ba92c0865742