staging: iio: ad7780: check if ad778x before gain update

Only the ad778x have the 'gain' status bit. Check it before updating
through a new variable is_ad778x in chip_info.

Signed-off-by: Giuliano Belinassi <giuliano.belinassi@usp.br>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>