[media] s5k6a3: Add missing entity function initialization

Suppresses warning like:
s5p-fimc-md camera: Entity type for entity S5K6A3 13-0010 was not initialized!

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