media: vdec: fix some kernel-doc warnings

Fix those warnings:
  drivers/media/platform/mtk-vcodec/vdec/vdec_h264_if.c:69: warning: No description found for parameter 'reserved'
  drivers/media/platform/mtk-vcodec/vdec/vdec_vp8_if.c:175: warning: Excess struct member 'dev' description in 'vdec_vp8_inst'

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>