iio: adc: ad7173: fix buffers enablement for ad7176-2

AD7176-2 does not feature input buffers and marks corespondent register
bits as read only. Enable buffers only on supported models.

Fixes: 76a1e6a4

 ("iio: adc: ad7173: add AD7173 driver")
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Dumitru Ceclan <dumitru.ceclan@analog.com>
Link: https://lore.kernel.org/r/20240521-ad7173-fixes-v1-1-8161cc7f3ad1@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>