media: v4l2-subdev: Guard whole fops and ioctl hdlr

A sub-device device node can be registered in user space only if the
CONFIG_VIDEO_V4L2_SUBDEV_API Kconfig option is selected. Currently the
open/close file operations and the ioctl handler have some parts of
their implementations guarded by #if
defined(CONFIG_VIDEO_V4L2_SUBDEV_API), while they are actually not
accessible without a video device node registered to user space.

Guard the whole open, close and ioctl handler and provide stubs if the
VIDEO_V4L2_SUBDEV_API Kconfig option is not selected.

This slightly reduces the kernel size when the option is not selected
and simplifies the file ops and ioctl implementations.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>