ALSA: hda - Improve loopback path lookups for AD1983

AD1983 has flexible loopback routes and the generic parser would take
wrong path confusingly instead of taking individual paths via NID 0x0c
and 0x0d.  For avoiding it, limit the connections at these widgets so
that the parser can think more straightforwardly.  This fixes the
regression of the missing line-in loopback on Dell machine.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=70011
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>