media: sun6i-csi: Refactor main driver data structures

Merge contents of structs sun6i_csi and sun6i_csi_dev into a main
sun6i_csi_device structure holding a sun6i_csi_v4l2 struct for things
related to v4l2, as well as the already-existing sun6i_csi_video and
sun6i_csi_config which are left unchanged.

This mostly simplifies accessing stuff by having a single main
structure accessible to every part of the code instead of a private
definition.

Also solve some kerneldoc warnings by describing return codes while
at it.

No functional change is intended in this commit, variables are just
moved around (cosmetics).

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>