iio: adc: ad7173: rename odr field

iio: adc: ad7173: rename odr field

Rename odr to sinc5_odr_index in the channel setup structure. In a
following commit, we will be adding a separate odr field for when the
sinc3 filter is used instead so having sinc5 in the name will help
avoid confusion. And _index makes it more clear that this is an index
of the sinc5_data_rates array and not the output data rate itself.

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>