iio: dac: adi-axi-dac: fix bus read

iio: dac: adi-axi-dac: fix bus read

Fix bus read function.

Testing the driver, on a random basis, wrong reads was detected, mainly
by a wrong DAC chip ID read at first boot.
Before reading the expected value from the AXI regmap, need always to
wait for busy flag to be cleared.

Fixes: e61d7178429a ("iio: dac: adi-axi-dac: extend features")
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250409-ad3552r-fix-bus-read-v2-1-34d3b21e8ca0@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>