ALSA: core: Remove child proc file elements recursively

This patch changes the way to manage the resource release of proc
files: namely, let snd_info_free_entry() freeing the whole children.

This makes it us possible to drop the snd_device_*() management.  Then
snd_card_proc_new() becomes merely a wrapper to
snd_info_create_card_entry().

Together with this change, now you need to call snd_info_free_entry()
for a proc entry created via snd_card_proc_new(), while it was freed
via snd_device_free() beforehand.

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