staging: iio: ad7780: add gain reading to ad778x

This patch adds a new functionality of reading gain values from the
ad778x chips. This value is stored in the chip's state struct and is
updated whenever a read or write call is performed on the driver.

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