media: sun6i-isp: proc: Fix return code handling in stream off path

Explicitly set ret to zero on disable path to avoid a related smatch
warning. This makes initialization at declaration useless.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Fixes: e3185e1d ("media: staging: media: Add support for the Allwinner A31 ISP")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>