media: staging: media: hantro: fix kernel-doc formatting

- @h264_dec was not documented, add that trivial change
- it is not possible to document a global variable in kernel-doc,
  change to use 'DOC:' instead
- fix mismatched struct name (hantro_vp8d_hw_ctx -> hantro_vp8_dec_hw_ctx)

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>