staging:iio:ad7780: add chip ID values and mask

The ad7780 supports both the ad778x and ad717x families. Each chip has
a corresponding ID. This patch provides a mask for extracting ID values
from the status bits and also macros for the correct values for the
ad7170, ad7171, ad7780 and ad7781.

Signed-off-by: Renato Lui Geh <renatogeh@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>