media: sun8i-a83t-mipi-csi2: Clarify return code handling in stream off path

Explicitly set ret to zero instead of assigning it and overwriting it
later, which is a bit confusing to understand.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>