The cobalt driver should depend on VIDEO_V4L2_SUBDEV_API.

This fixes this kbuild error:

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
head:   99bc7215
commit: 85756a06 [media] cobalt: add new driver
config: x86_64-randconfig-s0-09201514 (attached as .config)
reproduce:
  git checkout 85756a06
  # save the attached .config to linux build tree
  make ARCH=x86_64

All error/warnings (new ones prefixed by >>):

   drivers/media/i2c/adv7604.c: In function 'adv76xx_get_format':
>> drivers/media/i2c/adv7604.c:1853:9: error: implicit declaration of function 'v4l2_subdev_get_try_format' [-Werror=implicit-function-declaration]
      fmt = v4l2_subdev_get_try_format(sd, cfg, format->pad);
            ^
   drivers/media/i2c/adv7604.c:1853:7: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
      fmt = v4l2_subdev_get_try_format(sd, cfg, format->pad);
          ^
   drivers/media/i2c/adv7604.c: In function 'adv76xx_set_format':
   drivers/media/i2c/adv7604.c:1882:7: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
      fmt = v4l2_subdev_get_try_format(sd, cfg, format->pad);
          ^
   cc1: some warnings being treated as errors
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>