ALSA: 6fire: make buffers DMA-able (pcm)

Patch makes pcm buffers DMA-able by allocating each one separately.

Signed-off-by: Torsten Schenk <torsten.schenk@zoho.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>