mixer: Fix unused parameter warnings

mixer: Fix unused parameter warnings

mixer_simple.h:35:64: warning: unused parameter 'mixer' [-Wunused-parameter]
 static inline int snd_mixer_simple_basic_register(snd_mixer_t *mixer,
                                                                ^
mixer_simple.h:36:40: warning: unused parameter 'options' [-Wunused-parameter]
         struct snd_mixer_selem_regopt *options,
                                        ^
mixer_simple.h:37:29: warning: unused parameter 'classp' [-Wunused-parameter]
         snd_mixer_class_t **classp)
                             ^

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