iio: dac: ad5686: create bus ops struct

iio: dac: ad5686: create bus ops struct

Create struct with bus operations, which will be used to extend bus
implementation features. Auxiliary functions ad5686_write() and
ad5686_read() are created and ad5686_probe() now receives an ops struct
pointer rather than individual read and write functions.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Rodrigo Alencar <rodrigo.alencar@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>