staging:iio:adis16400: Fix and cleanup 3db filter setting

The 3db divisors table is partially wrong and incomplete. Also the code rounds
up to the next higher frequency if the requested frequency would matches one of
the available frequencies. These two issues are fixed by this patch. The patch
also changes the driver to round down the filter frequency if it is larger than
the largest supported frequency instead of rejecting it as an invalid value.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>