ALSA: hda/realtek - Improve the signel-connection check

When the connections from the pin selector contain only two
widgets, a route to DAC and the aa-mixer, it's certainly a
single connection.  In such a case, get_dac_if_single() should
return the connected DAC, too.

This will improve the detection of the individual DAC
assignment for each pin.

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