media: rcar-csi2: Fix comment of VCDT/VCDT2 register

According to latest H/W manual v1.50, the description of channel
number in the VCDT/VCDT2 register is decremented by one.
Therefore, this patch fixes it about comment.

Signed-off-by: Koji Matsuoka <koji.matsuoka.xm@renesas.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-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>