iio: adc: ti-ads8344: Fix channel selection

During initial submission the selection of the channel was done using
the scan_index member of the iio_chan_spec structure. It was an abuse
because this member is supposed to be used with a buffer so it was
removed.

However there was still the need to be able to known how to select a
channel, the correct member to store this information is address.

Thanks to this it is possible to select any other channel than the
channel 0.

Fixes: 8dd2d7c0

 ("iio: adc: Add driver for the TI ADS8344 A/DC chips")
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>