iio: filter: admv8818: fix integer overflow

HZ_PER_MHZ is only unsigned long. This math overflows, leading to
incorrect results.

Fixes: f34fe888 ("iio:filter:admv8818: add support for ADMV8818")
Signed-off-by: Sam Winchenbach <swinchenbach@arka.org>
Link: https://patch.msgid.link/20250328174831.227202-4-sam.winchenbach@framepointer.org


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