[media] v4l: omap4iss: Make __iss_video_get_format() return a v4l2_mbus_framefmt

The function will be used by a caller that needs the media bus format
instead of the pixel format currently returned. Move the media bus
format to pixel format conversion to the existing caller.

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