AD codecs have strange implementations for choosing the SPDIF-output
mux source: the digital audio out widget may take the sources from
multiple connections, where 0x01 indicates it's a PCM while others
point ADCs.  It's obviously invalid in the HD-audio spec POV, but it's
somehow convincing, too.  And, to make things more complex, AD1988A
and AD1882 have deeper connection routes that aren't expressed
correctly.

In this patch, the SPDIF mux control is implemented in two ways:
- For easier one like AD1981, AD1983, AD1884 and AD1984, where the
  SPDIF audio out widget takes just two or three sources, we can
  simply implement via the normal input_mux and connection verb
  calls.

- For the complex routes like AD1988A (but not AD1988B) or AD1882, we
  prepare "faked" paths represented statically, and switch the paths
  using these static ones, instead of parsing the routes from the
  widget tree.
Signed-off-by: NTakashi Iwai <tiwai@suse.de>