sound: dmasound_atari: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: m68k):

sound/oss/dmasound/dmasound_atari.c: warning: this statement may fall
through [-Wimplicit-fallthrough=]:  => 1449:24

Notice that, in this particular case, the code comment is
modified in accordance with what GCC is expecting to find.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>