ALSA: soc - wm9712 mono mixer

this fixes typo in wm9712 codec which prevents it from registering all audio
routes (and thus working correctly). Please consider applying.

(Tested and works on palmtx, palmld and palmt5)

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Mark Brown <brooie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>