iio: adc: ad4130: Adapt internal names to match official filter_type ABI

iio: adc: ad4130: Adapt internal names to match official filter_type ABI

Recently the interface to to select a filter was officially blessed to
use "filter_type". Adapt the naming of several functions accordingly to
make the new standard more present and so make the driver a better
template for other drivers. Apart from the comment update this is just
s/filter_mode/filter_type/.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://patch.msgid.link/20250303114659.1672695-15-u.kleine-koenig@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>