CHROMIUM: iio: cros_ec: use core mutex in sensors_ring

We have to use the mutex as we can suspend the machine while we are
processing a FIFO event from the EC.

BUG=chromium:711693
TEST=using next patch, run AIDA64 sensor page while enabling/disabling
ring bufffer. With mutex #defined out, kernel crashes.

Change-Id: I2a70f56a8e4965b6e0d409a8b4ded05dd75f78c7
Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/571416
Reviewed-by: Douglas Anderson <dianders@chromium.org>
(cherry picked from commit 9a2a8ba8fdfe091e2d39916cf69adbbba964be5c)
Reviewed-on: https://chromium-review.googlesource.com/586890
Tested-by: Douglas Anderson <dianders@chromium.org>