iio:adis16400: Add support for the 52.85 Hz base sampling rate

The adis16400 and similar have two different base sampling rate available, from
which the actual sampling rate is derived. 1638 Hz and 52.85 Hz, switching to
the lower base sampling rate allows to support lower sampling rates.

This patch adds support for switching to the lower base sampling rate if the
requested sampling frequency is outside of the range which can be supported by
the higher base sampling rate.

The function which is used to read the current sampling rate already has support
for the lower sampling rate, so no changes are required there.

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