ALSA: hda - Clear the dropped paths properly

When a DAC is reassigned from surrounds to front or ADCs are reduced
due to incomplete imux, we clear the path indices but the path
instances remain as is.  Since the paths might be still referred
through the whole path list parsing (e.g. is_active_nid()), we should
clear these path instances as well.

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