staging: iio: ad7780: add gain reading to ad778x

This patch adds a new functionality of reading gain values from the
ad778x chips. This value is stored in the chip's state struct and is
updated whenever a read or write call is performed on the driver.
Signed-off-by: Renato Lui Geh <renatogeh@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/adc/ad7780.c

@@ -42,6 +42,8 @@
 #define AD7780_GAIN_MIDPOINT	64
 #define AD7780_FILTER_MIDPOINT	13350
 
+static const unsigned int ad778x_gain[2] = { 1, 128 };
+
 struct ad7780_chip_info {
 	struct iio_chan_spec	channel;
 	unsigned int		pattern_mask;
@@ -179,6 +181,9 @@ static int ad7780_postprocess_sample(struct ad_sigma_delta *sigma_delta,
 	    ((raw_sample & chip_info->pattern_mask) != chip_info->pattern))
 		return -EIO;
 
+	if (chip_info->is_ad778x)
+		st->gain = ad778x_gain[raw_sample & AD7780_GAIN];
+
 	return 0;
 }