media: mtk-jpeg: Fix timeout schedule error in mtk_jpegdec_worker.
In mtk_jpegdec_worker, if error occurs in mtk_jpeg_set_dec_dst, it
will start the timeout worker and invoke v4l2_m2m_job_finish at
the same time. This will break the logic of design for there should
be only one function to call v4l2_m2m_job_finish. But now the timeout
handler and mtk_jpegdec_worker will both invoke it.
Fix it by start the worker only if mtk_jpeg_set_dec_dst successfully
finished.