[media] v4l2-subdev.h: fix v4l2_subdev_pad_config documentation

The fields of v4l2_subdev_pad_config are not pointers.

Fixes: 21c29de1 ("[media] v4l2-subdev.h: Improve documentation")

Cc: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>