iio: filter: admv8818: Support frequencies >= 2^32

iio: filter: admv8818: Support frequencies >= 2^32

This patch allows writing u64 values to the ADMV8818's high and low-pass
filter frequencies. It includes the following changes:

- Rejects negative frequencies in admv8818_write_raw.
- Adds a write_raw_get_fmt function to admv8818's iio_info, returning
  IIO_VAL_INT_64 for the high and low-pass filter 3dB frequency channels.

Fixes: f34fe888ad05 ("iio:filter:admv8818: add support for ADMV8818")
Signed-off-by: Brian Pellegrino <bpellegrino@arka.org>
Signed-off-by: Sam Winchenbach <swinchenbach@arka.org>
Link: https://patch.msgid.link/20250328174831.227202-7-sam.winchenbach@framepointer.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>