iio:adis16400: Increase samplerate precession

The devices supported by this drivers support sample rates with less than one
sample per second. To support this increase the samplerate precession to allow
setting (and reading) the samplerate with a milli-HZ precession.

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