ALSA: emu10k1: make tone control switch mono

It controls the whole surround set, so stereo can't work. As a
consequence, only the left channel was paid attention to.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Link: https://lore.kernel.org/r/20230510173917.3073107-2-oswald.buddenhagen@gmx.de


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