iio: adc: ad7923: fix channel readings for some variants

iio: adc: ad7923: fix channel readings for some variants

Some of the supported devices have 4 or 2 LSB trailing bits that should
not be taken into account. Hence we need to shift these bits out which
fits perfectly on the scan type shift property. This change fixes both
raw and buffered reads.

Fixes: f2f7a449707e ("iio:adc:ad7923: Add support for the ad7904/ad7914/ad7924")
Fixes: 851644a60d20 ("iio: adc: ad7923: Add support for the ad7908/ad7918/ad7928")
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220912081223.173584-2-nuno.sa@analog.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>