ALSA: hda/conexant - Enable stereo mix input for CX20549 and CX20551

Both CX20549 and CX20551 codecs have a mixer widget and it can be
connected as the ADC source.  Like AD and VIA codecs, enable the
add_stereo_mix_input flag for these codecs.

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