iio: adc: max1027: Fix wrong shift with 12-bit devices

10-bit devices must shift the value twice.
This is not needed anymore on 12-bit devices.

Fixes: ae47d009

 ("iio: adc: max1027: Introduce 12-bit devices support")
Cc: stable@vger.kernel.org
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210818111139.330636-2-miquel.raynal@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>