media: qcom: camss: Comment CSID dt_id field

Digging into the documentation we find that the DT_ID bitfield is used to
map the six bit DT to a two bit ID code. This value is concatenated to the
VC bitfield to create a CID value. DT_ID is the two least significant bits
of CID and VC the most significant bits.

Originally we set dt_id = vc * 4 in and then subsequently set dt_id = vc.

commit 3c4ed72a ("media: camss: sm8250: Virtual channels for CSID")
silently fixed the multiplication by four which would give a better
value for the generated CID without mentioning what was being done or why.

Next up I haplessly changed the value back to "dt_id = vc * 4" since there
didn't appear to be any logic behind it.

Hans asked what the change was for and I honestly couldn't remember the
provenance of it, so I dug in.

Link: https://lore.kernel.org/linux-arm-msm/edd4bf9b-0e1b-883c-1a4d-50f4102c3924@xs4all.nl/



Add a comment so the next hapless programmer doesn't make this same
mistake.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>