media: v4l2-fwnode: Make bus configuration a struct

The bus specific parameters were a union. This made providing bus specific
defaults impossible as the memory used to store the defaults for multiple
different busses was the same.

Make it struct instead. It's not large so the size isn't really an issue.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>