ALSA: korg1212: Convert to generic PCM copy ops

This patch converts the korg1212 driver code to use the new unified
PCM copy callback.  The open-coded conditional memory copies are
replaced with simpler copy_from/to_iter() calls.

Note that copy_from/to_iter() returns the copied bytes, hence the
error condition is adjusted accordingly.

Link: https://lore.kernel.org/r/20230815190136.8987-9-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>