ALSA: emu10k1: constify snd_emux_operators structure

Declare snd_emux_operators structure as const as it is only copied into
another structure. So, snd_emux_operators structures having this property
can be made const.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>