iio: cros_ec: Report hwfifo_watermark_max

Report the maximum amount of sample the EC can hold.
This is not tunable, but can be useful for application to find out the
maximum amount of time it can sleep when hwfifo_timeout is set to a
large number.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>