[ALSA] AMD Au1x00: fix DMA init/cleanup

Modules: MIPS AU1x00 driver

AMD Au1x00 ALSA driver causes kernel oops in au1000_init() by trying
to set DMA channel to -1 in yet unallocated audio streams. Here's the
patch that staightens up DMA init/cleanup code.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>