ALSA: ice1724 - limit channels for multi-channel playback

Limit the available channels for multi-channel playback device to
the real number of channels.  Until now, always up to 8 channels
are created, which are simply useless without the real outputs.

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