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

Patch makes midi output buffer DMA-able by allocating it separately.

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