iio: dac: ad3552r-hs: use instruction mode for configuration

iio: dac: ad3552r-hs: use instruction mode for configuration

Use "instruction" mode over initial configuration and all other
non-streaming operations.

DAC boots in streaming mode as default, and the driver is not
changing this mode.

Instruction r/w is still working because instruction is processed
from the DAC after chip select is deasserted, this works until
loop mode is 0 or greater than the instruction size.

All initial operations should be more safely done in instruction
mode, a mode provided for this.

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250114-wip-bl-ad3552r-axi-v0-iio-testing-carlos-v4-6-979402e33545@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>