media: v4l: i2c: Add a comment not to use static sub-device names in the future

A number of sub-device drivers used a static name for the sub-device, and
thus the media entity. As the entity name must be unique within a media
device, this makes it impossible to have more than one instance of each
device in a media device. This is a rather severe limitation.

Instead of fixing these drivers, add a comment to the drivers noting that
such static names may not be used in the future.

The alternative of fixing the drivers is troublesome as the entity (as
well as sub-device) name is part of the uAPI. Changing that is almost
certain to break something. As these devices are old but no-one has
encountered a problem with the static names, leave it as-is.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>