iio: adc: ad7606: fix wrong scale available

Fix wrong scale available list since only one value is returned:

...
iio:device1: ad7606b (buffer capable)
    8 channels found:
           voltage0:  (input, index: 0, format: le:S16/16>>0)
           2 channel-specific attributes found:
                 attr  0: scale value: 0.305176
                 attr  1: scale_available value: 0.076293
Fix as:
           voltage0:  (input, index: 0, format: le:S16/16>>0)
           2 channel-specific attributes found:
                 attr  0: scale value: 0.305176
                 attr  1: scale_available value: 0.076293 0.152588 0.305176

Fixes: 97c6d857 ("iio: adc: ad7606: rework scale-available to be static")
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250129-wip-bl-ad7606_add_backend_sw_mode-v3-3-c3aec77c0ab7@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>