iio: potentiometer: ds1803: Add available functionality

iio: potentiometer: ds1803: Add available functionality

Adding available functionality for DS1803 driver which
will show the minimum, step and maximum values that the
driver can accepts through sysfs entry.
Now using the max value present in avail array instead of chip
type specific macro to make the driver flexible to add other
type of potentiometer with different max position value.

Signed-off-by: Jagath Jog J <jagathjog1996@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220223163525.13399-3-jagathjog1996@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>