ALSA: hda - Fix bdl_pos_adj value for ATI SB chipsets

ATI SB controllers seem to report the DMA ahead in the amount of FIFO.
Thus bdl_pos_adj should be 32 for them as default.

Also, the default value is set to -1, which means to make the driver
to choose the appropriate value.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>