iio: dac: ad5446: Move to single chip_info structures

iio: dac: ad5446: Move to single chip_info structures

Do not use an array with an enum id kind of thing. Use the more
maintainable chip_info variable per chip.

Adapt the probe functions to use the proper helpers (for SPI and I2c).
Note that in a following patch we'll also add the chip_info variables to
the of_device_id tables. Hence already use the helpers that internally use
device_get_match_data().

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>