iio: addac: ad74413r: update channel function set

iio: addac: ad74413r: update channel function set

According to the datasheet switching to a new channel function implies
multiple steps.

All functions must be selected for a minimum of 130 μs before changing
to another function.
The DAC_CODEx registers are not reset by changing channel functions.
Prior to changing channel functions, it is recommended to set the DAC
code to 0x0000 via the DAC_CODEx registers. Set the channel function
to high impedance via the CH_FUNC_SETUPx registers before transitioning
to the new channel function. After the new channel function is configured,
it is recommended to wait 150 μs before updating the DAC code.

Even though the channel switch is done at only at probe, where a device
reset is performed, that does not guarantee that the steps prior
changing to new channel function should be neglected.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://lore.kernel.org/r/20230911142950.216687-1-antoniu.miclaus@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>