media: davinci: fix incorrect pix_fmt assignment

There's a mistakenly written self assignment in
`static int vpfe_enum_fmt_vid_cap(..)`.

Fixed it according to Prabhakar Lad's feedback.

Signed-off-by: Cengiz Can <cengiz@kernel.wtf>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>