iio: dac: ad7303: fix channel description

realbits, storagebits and shift should be numbers, not ASCII characters.

Signed-off-by: Pavel Roskin <plroskin@gmail.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>