iio: potentiometer: ds1803: Add device specific read_raw function

Added function pointer in the device specific structure to
call the appropriate device read_raw function, so that the
other type of devices with different read method can be
handled.

Signed-off-by: Jagath Jog J <jagathjog1996@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220223163525.13399-6-jagathjog1996@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>