staging/atomisp: add VIDEO_V4L2_SUBDEV_API dependency

The driver fails to build if this is disabled, so we need an explicit
Kconfig dependency:

drivers/staging/media/atomisp/pci/atomisp2/./atomisp_cmd.c:6085:48: error: 'struct v4l2_subdev_fh' has no member named 'pad'

Fixes: a49d2536 ("staging/atomisp: Add support for the Intel IPU v2")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>