[ALSA] Fix noisy output with shared channel mode with hd-audio

- Fix the wrong initialization of num_dacs when changing the channel
  mode between 2 and multi-channel modes.  It must be evaluated
  after calling snd_hda_ch_mode_put()
- Added the similar check of num_dacs fix in Realtek code.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>