ALSA: control: Fix breakage of user ctl element addition

In the commit [2225e79b: 'ALSA: core: reduce stack usage related
to snd_ctl_new()'], the id field of the newly added kctl is untouched,
thus all attribute like name string remain empty.  The fix is just to
add the forgotten memcpy of the id field.

Fixes: 2225e79b

 ('ALSA: core: reduce stack usage related to snd_ctl_new()')
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>