[ALSA] cs5535audio: correctly set dma->substream

[ALSA] cs5535audio: correctly set dma->substream

We're never actually setting dma->substream to the current substream; that
means the dma->substream checks that we do in the suspend/resume path
are never satisfied, and the PRD registers are never correctly managed.  This
changes it so that we set the substream when constructing the specific
bus master DMA, and unsetting it when we tear down the BM's DMA.

Signed-off-by: Andres Salomon <dilinger@debian.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>