ALSA: wm8750: it's MONO1, not MONO

Since first commit wm8750 contained output named MONO, but
all routes mentioned MONO1. Correct MONO to be MONO1.

Signed-off-by: Dmitry Baryshkov <dbaryshkov@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>