[media] v4l2-subdev: Fix compilation when !VIDEO_V4L2_SUBDEV_API

[media] v4l2-subdev: Fix compilation when !VIDEO_V4L2_SUBDEV_API

As reported by Kbuildtest:

   drivers/media/v4l2-core/v4l2-subdev.c: In function 'check_format':
   drivers/media/v4l2-core/v4l2-subdev.c:136:23: error: 'struct v4l2_subdev' has no member named 'entity'
     if (format->pad >= sd->entity.num_pads)
                          ^
   drivers/media/v4l2-core/v4l2-subdev.c: In function 'check_crop':
   drivers/media/v4l2-core/v4l2-subdev.c:148:21: error: 'struct v4l2_subdev' has no member named 'entity'
     if (crop->pad >= sd->entity.num_pads)
                        ^
   drivers/media/v4l2-core/v4l2-subdev.c: In function 'check_selection':
   drivers/media/v4l2-core/v4l2-subdev.c:161:20: error: 'struct v4l2_subdev' has no member named 'entity'
     if (sel->pad >= sd->entity.num_pads)
                       ^
   drivers/media/v4l2-core/v4l2-subdev.c: In function 'check_edid':
   drivers/media/v4l2-core/v4l2-subdev.c:169:21: error: 'struct v4l2_subdev' has no member named 'entity'
     if (edid->pad >= sd->entity.num_pads)
                        ^
   drivers/media/v4l2-core/v4l2-subdev.c: In function 'subdev_do_ioctl':
>> drivers/media/v4l2-core/v4l2-subdev.c:186:6: warning: unused variable 'rval' [-Wunused-variable]
     int rval;
         ^
   drivers/media/v4l2-core/v4l2-subdev.c: At top level:
   drivers/media/v4l2-core/v4l2-subdev.c:129:12: warning: 'check_format' defined but not used [-Wunused-function]
    static int check_format(struct v4l2_subdev *sd,
               ^
   drivers/media/v4l2-core/v4l2-subdev.c:142:12: warning: 'check_crop' defined but not used [-Wunused-function]
    static int check_crop(struct v4l2_subdev *sd, struct v4l2_subdev_crop *crop)
               ^
   drivers/media/v4l2-core/v4l2-subdev.c:154:12: warning: 'check_selection' defined but not used [-Wunused-function]
    static int check_selection(struct v4l2_subdev *sd,
               ^
   drivers/media/v4l2-core/v4l2-subdev.c:167:12: warning: 'check_edid' defined but not used [-Wunused-function]
    static int check_edid(struct v4l2_subdev *sd, struct v4l2_subdev_edid *edid)

The above warnins happen because those functions are used only
when the V4L2 subdev API is enabled.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>