ALSA: dmaengine: Synchronize dma channel in prepare()

commit 929fa0c11b912fc594eaada342579cf56fef4d1c from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git



Sometimes the stream may be stopped due to XRUN events, in which case
the userspace can call snd_pcm_drop() and snd_pcm_prepare() to stop and
start the stream again.

In these cases, we must wait for the DMA channel to synchronize before
marking the stream as prepared for playback, as the DMA channel gets
stopped by snd_pcm_drop() without any synchronization.

Signed-off-by: Jai Luthra <j-luthra@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>