iio: potentiometer: ds1803: Add channel information in device data

iio: potentiometer: ds1803: Add channel information in device data

Adding each device wiper count and channel information into
device private data.
Utilizing addr member of struct iio_chan_spec to get the
wiper register address so that the value can be read or write
to the same address.

Signed-off-by: Jagath Jog J <jagathjog1996@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220223163525.13399-4-jagathjog1996@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>