iio: addac: ad74413r: fix Current Input, Loop Powered Mode

iio: addac: ad74413r: fix Current Input, Loop Powered Mode

Currently, the driver handles CH_FUNC_CURRENT_INPUT_LOOP_POWER and
CH_FUNC_CURRENT_INPUT_EXT_POWER completely identically. But that's not
correct. In order for CH_FUNC_CURRENT_INPUT_LOOP_POWER to work, two
changes must be made:

(1) expose access to the DAC_CODE_x register so that the intended
output current can be set, i.e. expose the channel as both current
output and current input, and

(2) per the data sheet

  When selecting the current input loop powered function, tie the
  VIOUTN_x pin to ground via the on-chip 200 kΩ resistor by enabling
  the CH_200K_TO_GND bit in the ADC_CONFIGx registers.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20230301115511.849418-1-linux@rasmusvillemoes.dk
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>