ad7780: fix division by zero in ad7780_write_raw()

ad7780: fix division by zero in ad7780_write_raw()

In the ad7780_write_raw() , val2 can be zero, which might lead to a
division by zero error in DIV_ROUND_CLOSEST(). The ad7780_write_raw()
is based on iio_info's write_raw. While val is explicitly declared that
can be zero (in read mode), val2 is not specified to be non-zero.

Fixes: 9085daa4abcc ("staging: iio: ad7780: add gain & filter gpio support")
Cc: stable@vger.kernel.org
Signed-off-by: Zicheng Qu <quzicheng@huawei.com>
Link: https://patch.msgid.link/20241028142027.1032332-1-quzicheng@huawei.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>