mfd / platform / iio: cros_ec: Register sensor through sensorhub

Remove the duplicated code in MFD, since MFD just registers cros-ec-sensorhub
if at least one sensor is present.

Change IIO cros-ec driver to get the pointer to the cros-ec-dev through
cros-ec-sensorhub.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>