staging:iio:adis16400: Don't pass 0 to ilog2

ilog2 is not defined for 0, so we need to handle the case where the requested
frequency is larger than the base sampling rate. In this case we'll round down
and set the sampling rate to the base sampling rate.

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