media: sun6i-isp: capture: Fix uninitialized variable use

While the stride_chroma variable was previously initialized to zero,
it's actually stride_chroma_div4 that is set to hardware registers.

Initialize it to zero instead to avoid an uninitialized variable use
and get rid of the associated smatch warning.

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>