staging: iio: ad7816: Switch to the gpio descriptor interface

Use the gpiod interface for rdwr_pin, convert_pin and busy_pin
instead of the deprecated old non-descriptor interface.

Note that Phil Reid identified that some of this code would never
have worked.  A follow up patch can fix that as this is just
a direct change of interface.

Signed-off-by: Nishad Kamdar <nishadkamdar@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>