Don't break the loop in snd_hda_codec_build_pcms() even if the item
has no substreams.

It's possible that it's an empty item and the next item containing
the valid substreams (e.g. realtek codecs may create the analog
and alt-analog but no digitl streams).
Signed-off-by: Takashi Iwai <tiwai@suse.de>