ALSA: korg1212: Add __force annotation to cast in user-copy callbacks

The user-copy callbacks in korg1212 driver contain the explicit cast
from a user pointer to a kernel pointer, but they missed __force
prefix.  It's mandatory for converting between them.

Spotted by sparse, a warning like:
  sound/pci/korg1212/korg1212.c:1329:33: warning: cast removes address space of expression

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