iio: mxs-lradc: fix iio channel map regression

Since commit c8231a9a ("iio: mxs-lradc: compute temperature
from channel 8 and 9") with the removal of adc channel 9 there is
no 1-1 mapping in the channel spec.

All hwmon channel values above 9 are accessible via there index minus
one. So add a hidden iio channel 9 to fix this issue.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Cc: Stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>