[media] v4l: s5k6a3: Return V4L2_FIELD_NONE from pad-level set format

The sensor is progressive, always return the field order set to
V4L2_FIELD_NONE.

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>