[ALSA] sparc dbri: OSS layer fix

This patch removes setting of incorrect stop_threshold value
inside the driver. After the change, playback through the OSS
layer works correctly.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>