[ALSA] hda-codec - alc268 input_mux should be a selector instead of mixer

According to the [0003659], the node 0x23,0x24 is a selector.
I checked the alc268 spec on the REALTEK website and it showed that they
were selectors indeed.
However, current code implement the alc268 input_mux in a mixer way.

Signed-off-by: Jiang Zhe <zhe.jiang@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>