media: staging: rkisp1: rsz: remove redundant if statement and add inline doc

The statement "if (rsz->fmt_type == V4L2_PIXEL_ENC_YUV)"
can be removed since the value of rsz->fmt_type is either
V4L2_PIXEL_ENC_YUV or V4L2_PIXEL_ENC_BAYER and the function
returns if it is bayer. In addition some doc with clarification
is added.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>