ad7780: fix division by zero in ad7780_write_raw()

In the Linux kernel, the following vulnerability has been resolved:

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.