iio: bmg160: add callbacks for the filter frequency

The filter frequency and sample rate have a fixed relationship.
Only the filter frequency is unique, however.
Currently the driver ignores the filter settings for 32 Hz and
64 Hz.

This patch adds the necessary callbacks to be able to configure
and read the filter setting from sysfs.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>