sound/sound_core: Fix sparse warnings

Fix the following sparse warnings:

sound/sound_core.c:460:2: warning: returning void-valued expression
sound/sound_core.c:477:2: warning: returning void-valued expression
sound/sound_core.c:510:5: warning: symbol 'soundcore_open' was not
declared. Should it be static?

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>