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