ALSA: Au88x0 - Fix IRQ fifo error and channels swap of 4 channels playback

Fix IRQ fifo error when playing stereo by set stereo flag of fifo control.
This also fix the swap of front and rear channels on au8830.

Signed-off-by: Raymond Yau <superquad.vortex2@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>