staging: iio: ad7780: fix offset read value

Variable val subtracted an uninitialized value on IIO_CHAN_INFO_OFFSET.
This was fixed by assigning the correct value instead.

Signed-off-by: Renato Lui Geh <renatogeh@gmail.com>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>