ALSA: hda - Disable SPDIF only when no pin config set for HP with AD1981

Some HP laptops with AD1981 have SPDIF connections, but currently the
driver disables it statically.  Better to check the pin default config
to judge whether to enable or disable the SPDIF.

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