iio: adc: ad4695: rename AD4695_MAX_VIN_CHANNELS

Rename AD4695_MAX_CHANNELS to AD4695_MAX_VIN_CHANNELS. It has been a
point of confusion that this macro is only the voltage input channels
and not all channels.

Reviewed-by: Trevor Gamblin <tgamblin@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250507-iio-introduce-iio_declare_buffer_with_ts-v6-4-4aee1b9f1b89@baylibre.com


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