ALSA: info: Always register entries recursively

Make sure that all children entries are registered by a single call of
snd_info_register().  OTOH, don't register if a parent isn't
registered yet.

This allows us to create the whole procfs tree in a shot at the last
stage of card registration phase in a later patch.

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