ALSA: control: Keep the previous numid at snd_ctl_rename_id()

We don't need to change the numid at each time snd_ctl_rename_id() is
called, as the control element size itself doesn't change.  Let's keep
the previous numid value.

Along with it, add a note about calling this function only in the
card init phase.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230606094035.14808-1-tiwai@suse.de


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