staging: iio: ad2s1210: Fix SPI reading

If the serial interface is used, the 8-bit address should be latched using
the rising edge of the WR/FSYNC signal.

This basically means that a CS change is required between the first byte
sent, and the second one.
This change splits the single-transfer transfer of 2 bytes into 2 transfers
with a single byte, and CS change in-between.

Note fixes tag is not accurate, but reflects a point beyond which there
are too many refactors to make backporting straight forward.

Fixes: b19e9ad5e2 ("staging:iio:resolver:ad2s1210 general driver cleanup.")
Signed-off-by: Dragos Bogdan <dragos.bogdan@analog.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/resolver/ad2s1210.c

@@ -130,17 +130,24 @@ static int ad2s1210_config_write(struct ad2s1210_state *st, u8 data)
 static int ad2s1210_config_read(struct ad2s1210_state *st,
 				unsigned char address)
 {
-	struct spi_transfer xfer = {
-		.len = 2,
-		.rx_buf = st->rx,
-		.tx_buf = st->tx,
+	struct spi_transfer xfers[] = {
+		{
+			.len = 1,
+			.rx_buf = &st->rx[0],
+			.tx_buf = &st->tx[0],
+			.cs_change = 1,
+		}, {
+			.len = 1,
+			.rx_buf = &st->rx[1],
+			.tx_buf = &st->tx[1],
+		},
 	};
 	int ret = 0;
 
 	ad2s1210_set_mode(MOD_CONFIG, st);
 	st->tx[0] = address | AD2S1210_MSB_IS_HIGH;
 	st->tx[1] = AD2S1210_REG_FAULT;
-	ret = spi_sync_transfer(st->sdev, &xfer, 1);
+	ret = spi_sync_transfer(st->sdev, xfers, 2);
 	if (ret < 0)
 		return ret;