staging: iio: resolver: ad2s1210: always use 16-bit value for raw read

This removes the special handling for resolutions lower than 16 bits.
This will allow us to use a fixed scale independent of the resolution.

A new sample field is added to store the raw data instead of reusing
the config mode rx buffer so that we don't have to do a cast or worry
about unaligned access.

Also, for the record, according to the datasheet, the logic for the
special handling based on hysteresis that was removed was incorrect.
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20230929-ad2s1210-mainline-v3-7-fa4364281745@baylibre.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/resolver/ad2s1210.c

@@ -95,7 +95,11 @@ struct ad2s1210_state {
 	bool hysteresis;
 	u8 resolution;
 	enum ad2s1210_mode mode;
-	u8 rx[2] __aligned(IIO_DMA_MINALIGN);
+	/** For reading raw sample value via SPI. */
+	__be16 sample __aligned(IIO_DMA_MINALIGN);
+	/** SPI transmit buffer. */
+	u8 rx[2];
+	/** SPI receive buffer. */
 	u8 tx[2];
 };
 
@@ -464,10 +468,7 @@ static int ad2s1210_read_raw(struct iio_dev *indio_dev,
 			     long m)
 {
 	struct ad2s1210_state *st = iio_priv(indio_dev);
-	u16 negative;
 	int ret = 0;
-	u16 pos;
-	s16 vel;
 
 	mutex_lock(&st->lock);
 	gpiod_set_value(st->gpios[AD2S1210_SAMPLE], 0);
@@ -487,26 +488,17 @@ static int ad2s1210_read_raw(struct iio_dev *indio_dev,
 	}
 	if (ret < 0)
 		goto error_ret;
-	ret = spi_read(st->sdev, st->rx, 2);
+	ret = spi_read(st->sdev, &st->sample, 2);
 	if (ret < 0)
 		goto error_ret;
 
 	switch (chan->type) {
 	case IIO_ANGL:
-		pos = be16_to_cpup((__be16 *)st->rx);
-		if (st->hysteresis)
-			pos >>= 16 - st->resolution;
-		*val = pos;
+		*val = be16_to_cpu(st->sample);
 		ret = IIO_VAL_INT;
 		break;
 	case IIO_ANGL_VEL:
-		vel = be16_to_cpup((__be16 *)st->rx);
-		vel >>= 16 - st->resolution;
-		if (vel & 0x8000) {
-			negative = (0xffff >> st->resolution) << st->resolution;
-			vel |= negative;
-		}
-		*val = vel;
+		*val = (s16)be16_to_cpu(st->sample);
 		ret = IIO_VAL_INT;
 		break;
 	default: