[ALSA] sparc dbri: hardware constrains added

This patch adds ALSA hardware constrains so stereo is possible
only with 16-bit format. It contains small cleanups to ring
buffered code as well.

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>