iio: adc: adi-axi-adc: remove regmap max register

iio: adc: adi-axi-adc: remove regmap max register

In one of the following patches, we'll have some new functionality that
requires reads/writes on registers bigger than 0x8000. Hence, as this is
an highly flexible core, don't bother in setting 'max_register' and
remove it from regmap_config.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240426-ad9467-new-features-v2-5-6361fc3ba1cc@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>