iio: adc: ad7944: mask high bits on direct read

Apply a mask to the raw value received over the SPI bus for unsigned
direct reads. As we found recently, SPI controllers may not set unused
bits to 0 when reading with bits_per_word != {8,16,32}. The ad7944 uses
bits_per_word of 14 and 18, so we need to mask the value to be sure we
returning the correct value to userspace during a direct read.

Fixes: d1efcf8871db ("iio: adc: ad7944: add driver for AD7944/AD7985/AD7986")
Signed-off-by: David Lechner <[e-mail]>
Reviewed-by: Nuno Sá <[e-mail]>
Link: https://patch.msgid.link/202505[e-mail]
Cc: <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/ad7944.c

···
 
 	if (chan->scan_type.sign == 's')
 		*val = sign_extend32(*val, chan->scan_type.realbits - 1);
+	else
+		*val &= GENMASK(chan->scan_type.realbits - 1, 0);
 
 	return IIO_VAL_INT;
 }