staging: iio: ad7816: Do not use busy_pin in case of AD7818

AD7818 does not support busy_pin functionality as per datasheet.
Hence drop busy_pin when AD7818 is used.

Signed-off-by: Nishad Kamdar <nishadkamdar@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>