Impact: Change signature of 'set_volume_stereo' and 'set_volume_mono'.

Fix this sparse warnings:
  sound/oss/pss.c:545:42: warning: incorrect type in argument 2 (different signedness)
  sound/oss/pss.c:546:42: warning: incorrect type in argument 3 (different signedness)
  sound/oss/pss.c:554:59: warning: incorrect type in argument 2 (different signedness)
  sound/oss/pss.c:560:59: warning: incorrect type in argument 2 (different signedness)
  sound/oss/pss.c:566:59: warning: incorrect type in argument 2 (different signedness)
Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>