ALSA: control: Fix missing __user annotation

There is one place missing __user annotation to the pointer used by
the recent code refactoring.  Reported by sparse.

Fixes: 450296f3

 ("ALSA: control: code refactoring TLV ioctl handler")
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>