media: v4l2-subdev.rst: correct information about v4l2 events

Remove description of non-existing v4l2_subdev.nevents and replace the
undefined flag V4L2_SUBDEV_USES_EVENTS by the correct flag
V4L2_SUBDEV_FL_HAS_EVENTS, which is already documented in v4l2_subdev.flags

Fixes: commit 02adb1cc

 ("[media] v4l: subdev: Events support")
Signed-off-by: Michael Rodin <mrodin@de.adit-jv.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>