[ALSA] hda-codec - Fix input_mux numbers for vaio stac92xx

My bad, I forgot to update the num_items field when added a new item
to vaio_mux items table, so the last item 'PCM' disappeared.
Now it has the right number 3.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>