media: i2c: video-i2c: Move defines to the top of the file

Currently, the defines in this driver are after some structs and
functions, it makes more sense to move them up to the top of the file,
so that the constants can be named together with other defines.

Signed-off-by: Moses Christopher Bollavarapu <mosescb.dev@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>