[media] si470x: fixup wait_for_completion_timeout return handling

return type of wait_for_completion_timeout is unsigned long not int. A
appropriately named variable of type unsigned long is added and the
assignments fixed up.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>