ALSA: gus: Convert to generic PCM copy ops

This patch converts the GUS driver code to use the new unified PCM
copy callback.  It's a straightforward conversion from *_user() to
*_iter() variants.

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

Link: https://lore.kernel.org/r/20230815190136.8987-6-tiwai@suse.de


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