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>

drivers/media/platform/mtk-vcodec/vdec/vdec_h264_if.c

@@ -59,6 +59,7 @@ struct h264_fb {
  * @read_idx  : read index
  * @write_idx : write index
  * @count     : buffer count in list
+ * @reserved  : for 8 bytes alignment
  */
 struct h264_ring_fb_list {
 	struct h264_fb fb_list[H264_MAX_FB_NUM];

drivers/media/platform/mtk-vcodec/vdec/vdec_vp8_if.c

@@ -155,7 +155,6 @@ struct vdec_vp8_vpu_inst {
  * @reg_base		   : HW register base address
  * @frm_cnt		   : decode frame count
  * @ctx			   : V4L2 context
- * @dev			   : platform device
  * @vpu			   : VPU instance for decoder
  * @vsi			   : VPU share information
  */