media: rcar-vin: Handle special pixel formats in a switch

Before extending the special pixel format handling turn the multiple if
statements into a switch.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>