iio: dac: ad5380: fix incorrect assignment to val

Currently the pointer val is being incorrectly incremented
instead of the value pointed to by val. Fix this by adding
in the missing * indirection operator.

Addresses-Coverity: ("Unused value")
Fixes: c03f2c53 ("staging:iio:dac: Add AD5380 driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>