iio: adc: ad7606: add missing max sample rates

iio: adc: ad7606: add missing max sample rates

Add max sample rates for all of the chips. Previously, only one chip had
this field populated. The fallback value for the initial sampling
frequency can be removed now that all chips have a max sample rate
defined.

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250318-iio-adc-ad7606-improvements-v2-3-4b605427774c@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>