media: sun6i-csi: Tidy up v4l2 code

Various cosmetic improvements to the v4l2 registration code, with
renames, lowerings, etc. The cleanup function is moved down after
setup. No functional change intended.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>