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

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: df1d80aee963 ("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>