[media] v4l: of: Instead of zeroing bus_type and bus field separately, unify this

Zero the entire struct starting from bus_type. As more fields are added, no
changes will be needed in the function to reset their value explicitly.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>