CHROMIUM: ARM: exynos: Avoid race if multiple thermistor readers at once

CHROMIUM: ARM: exynos: Avoid race if multiple thermistor readers at once

The adc code expects one client per process.

BUG=chrome-os-partner:14115
TEST=run the below cmd from 8 shells for 60 mins and saw no crash
  cd /sys/devices/platform/ncp15wb473.0/
  while true; do cat temp1_input; done

Change-Id: Ie34986648cbb7f43597391314719df5976937de0
Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/33645
Tested-by: Abhinav Kochhar <abhinav@chromium.org>
Reviewed-by: Vikas Sajjan <vikas.sajjan@samsung.com>
Reviewed-by: Abhinav Kochhar <abhinav@chromium.org>
Reviewed-by: Grant Grundler <grundler@chromium.org>