ALSA: hda - Add snd_hda_multi_out_dig_cleanup()

Added the helper function snd_hda_multi_out_dig_cleanup() to clean up
the digital outputs with multi setup.  This call is needed in cases
the codec supports multiple digital outputs as slaves.  Otherwise the
slave widgets aren't properly cleaned up.

For a single digital output (e.g. in patch_conexant.c), this call isn't
needed.

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