iio: adc: ad_sigma_delta: Handle CS assertion as intended in ad_sd_read_reg_raw()

When struct ad_sigma_delta::keep_cs_asserted was introduced only
register writing was adapted to honor this new flag. Also respect it
when reading a register.

Fixes: df1d80a ("iio: ad_sigma_delta: Properly handle SPI bus locking vs CS assertion")
Reviewed-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://patch.msgid.link/8d76b4ea4a4363b269886c71193b840821c724ea.1733504533.git.u.kleine-koenig@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Origin: next-20241216~37^2~21, commit:90b8b2fe60eb673d917b3c11abfc0a8ee144145e

Author: ukleinek

drivers/iio/adc/ad_sigma_delta.c

@@ -110,7 +110,7 @@ static int ad_sd_read_reg_raw(struct ad_sigma_delta *sigma_delta,
 		}, {
 			.rx_buf = val,
 			.len = size,
-			.cs_change = sigma_delta->bus_locked,
+			.cs_change = sigma_delta->keep_cs_asserted,
 		},
 	};
 	struct spi_message m;