ALSA: hda/realtek - Try harder to fit the single-connections

So far, the Realtek driver tires to assign the single-connected routes
for all pins only once at the beginning.  However, since some DACs have
been already mapped, the rest pins might have also single conections.

In this patch, the driver does the single-connection assignment in a
loop until all possbile single-connections are checked.  This will
improve the DAC assignment, e.g. for ASUS G72.

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