staging: iio: adt7316: fix the dac write calculation

The lsb calculation is not masking the correct bits from the user input.
Subtract 1 from (1 << offset) to correctly set up the mask to be applied
to user input.

The lsb register stores its value starting at the bit 7 position.
adt7316_store_DAC() currently assumes the value is at the other end of the
register. Shift the lsb value before storing it in a new variable lsb_reg,
and write this variable to the lsb register.

Fixes: 35f6b6b ("staging: iio: new ADT7316/7/8 and ADT7516/7/9 driver")
Signed-off-by: Jeremy Fertic <jeremyfertic@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/addac/adt7316.c

@@ -1431,7 +1431,7 @@ static ssize_t adt7316_show_DAC(struct adt7316_chip_info *chip,
 static ssize_t adt7316_store_DAC(struct adt7316_chip_info *chip,
 				 int channel, const char *buf, size_t len)
 {
-	u8 msb, lsb, offset;
+	u8 msb, lsb, lsb_reg, offset;
 	u16 data;
 	int ret;
 
@@ -1449,9 +1449,13 @@ static ssize_t adt7316_store_DAC(struct adt7316_chip_info *chip,
 		return -EINVAL;
 
 	if (chip->dac_bits > 8) {
-		lsb = data & (1 << offset);
+		lsb = data & ((1 << offset) - 1);
+		if (chip->dac_bits == 12)
+			lsb_reg = lsb << ADT7316_DA_12_BIT_LSB_SHIFT;
+		else
+			lsb_reg = lsb << ADT7316_DA_10_BIT_LSB_SHIFT;
 		ret = chip->bus.write(chip->bus.client,
-			ADT7316_DA_DATA_BASE + channel * 2, lsb);
+			ADT7316_DA_DATA_BASE + channel * 2, lsb_reg);
 		if (ret)
 			return -EIO;
 	}