[media] mtk_vcodec_dec: return error at mtk_vdec_pic_info_update()

[media] mtk_vcodec_dec: return error at mtk_vdec_pic_info_update()

Gcc 7.1 complains that:

	drivers/media/platform/mtk-vcodec/mtk_vcodec_dec.c: In function 'mtk_vdec_pic_info_update':
	drivers/media/platform/mtk-vcodec/mtk_vcodec_dec.c:284:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
	  int ret;
	      ^~~

Indeed, if debug is disabled, "ret" is never used. The best
fix for it seems to make the fuction to return an error code.

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