sound: sound/oss/dmasound/: cleanups

This patch contains the following cleanups:
- make the following needlessly global functions static:
  - dmasound_core.c: get_afmt_string()
  - dmasound_paula.c: dmasound_paula_init()
  - dmasound_q40.c: dmasound_q40_init()
- remove the following unused global variable:
  - dmasound_core.c: software_input_volume

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>