media: mc: Provide a helper for setting bus_info field

The bus_info or a similar field exists in a lot of structs, yet drivers
tend to set the value of that field by themselves in a determinable way.
Thus provide a helper for doing this. To be used in subsequent patches.

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@kernel.org>