sound: oss: dmasound: remove software_input_volume declaration

expand_read_bal has been removed since commit fc37449f ("The
next round of scheduled OSS code removal").

software_input_volume has been removed since
commit 0a1b42db ("sound: sound/oss/dmasound/: cleanups").

so remove the declare for them from header file.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Link: https://lore.kernel.org/r/20220909061126.1129585-1-cuigaosheng1@huawei.com


Signed-off-by: Takashi Iwai <tiwai@suse.de>