media: videodev2.h: Fix struct v4l2_input tuner index comment

VIDIOC_ENUMINPUT documentation describes the tuner field of
struct v4l2_input as index:

Documentation/userspace-api/media/v4l/vidioc-enuminput.rst
"
* - __u32
  - ``tuner``
  - Capture devices can have zero or more tuners (RF demodulators).
    When the ``type`` is set to ``V4L2_INPUT_TYPE_TUNER`` this is an
    RF connector and this field identifies the tuner. It corresponds
    to struct :c:type:`v4l2_tuner` field ``index``. For
    details on tuners see :ref:`tuner`.
"

Drivers I could find also use the 'tuner' field as an index, e.g.:
drivers/media/pci/bt8xx/bttv-driver.c bttv_enum_input()
drivers/media/usb/go7007/go7007-v4l2.c vidioc_enum_input()

However, the UAPI comment claims this field is 'enum v4l2_tuner_type':
include/uapi/linux/videodev2.h

This field being 'enum v4l2_tuner_type' is unlikely as it seems to be
never used that way in drivers, and documentation confirms it. It seem
this comment got in accidentally in the commit which this patch fixes.
Fix the UAPI comment to stop confusion.

This was pointed out by Dmitry while reviewing VIDIOC_ENUMINPUT
support for strace.

Fixes: 6016af82 ("[media] v4l2: use __u32 rather than enums in ioctl() structs")
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>