[media] am437x-vpfe: fix an uninitialized variable bug

If we are doing V4L2_FIELD_NONE then "ret" is used uninitialized.

Fixes: 417d2e50 ('[media] media: platform: add VPFE capture driver support for AM437X')

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>