iio: filter: admv8818: fix band 4, state 15

Corrects the upper range of LPF Band 4 from 18.5 GHz to 18.85 GHz per
the ADMV8818 datasheet

Fixes: f34fe888 ("iio:filter:admv8818: add support for ADMV8818")
Signed-off-by: Sam Winchenbach <swinchenbach@arka.org>
Link: https://patch.msgid.link/20250328174831.227202-3-sam.winchenbach@framepointer.org


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