staging:iio:ad7780: Mark channels as unsigned

The values reported by the AD7780 are unsigned with a binary offset:

	0x000000 is negative fullscale
	0x800000 is zeroscale
	0xffffff is positive fullscale

So mark the channel in the channel spec as unsigned rather than signed.
Signed-off-by: NLars-Peter Clausen <lars@metafoo.de>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

drivers/staging/iio/adc/ad7780.c

@@ -131,9 +131,10 @@ static const struct ad7780_chip_info ad7780_chip_info_tbl[] = {
 			.indexed = 1,
 			.channel = 0,
 			.info_mask = IIO_CHAN_INFO_RAW_SEPARATE_BIT |
-			IIO_CHAN_INFO_SCALE_SHARED_BIT,
+			IIO_CHAN_INFO_SCALE_SHARED_BIT |
+			IIO_CHAN_INFO_OFFSET_SHARED_BIT,
 			.scan_type = {
-				.sign = 's',
+				.sign = 'u',
 				.realbits = 24,
 				.storagebits = 32,
 				.shift = 8,
@@ -146,9 +147,10 @@ static const struct ad7780_chip_info ad7780_chip_info_tbl[] = {
 			.indexed = 1,
 			.channel = 0,
 			.info_mask = IIO_CHAN_INFO_RAW_SEPARATE_BIT |
-			IIO_CHAN_INFO_SCALE_SHARED_BIT,
+			IIO_CHAN_INFO_SCALE_SHARED_BIT |
+			IIO_CHAN_INFO_OFFSET_SHARED_BIT,
 			.scan_type = {
-				.sign = 's',
+				.sign = 'u',
 				.realbits = 20,
 				.storagebits = 32,
 				.shift = 12,