[media] v4l: of: Use of_get_child_by_name()

Replace a manual loop through child nodes with a call to
of_get_child_by_name().

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>