media: staging: media: starfive: Clean pad selection in isp_try_format()

The code to select isp_dev->formats[] is overly complicated.  We can
just use the "pad" as the index.  This will making adding new pads
easier in future patches.  No functional change.

Link: https://lore.kernel.org/linux-media/20240312024520.11022-1-changhuang.liang@starfivetech.com
Signed-off-by: Changhuang Liang <changhuang.liang@starfivetech.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>