[media] s5c73m3: Fix entity function assignment for the OIF subdev

Suppresses warnings like:
s5p-fimc-md camera: Entity type for entity S5C73M3-OIF was not initialized!

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>