iio: ad_sigma_delta: use unsigned long for timeout

wait_for_completion_timeout returns unsigned long not int so an appropriate
variable is declared and the assignment and check fixed up.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>