ALSA: sb: Fix wrong argument in commented code

While rewriting the code from sockptr_t to iov_iter during the
development, I forgot to replace one place in emu8000-pcm code.  As
it's in the disabled area (with ifdef), it's never built and
overlooked.  Replace with the proper argument NULL.

Fixes: 9d0fdc60 ("ALSA: emu8000: Convert to generic PCM copy ops")
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Closes: https://lore.kernel.org/r/20230902053646.GK3390869@ZenIV
Link: https://lore.kernel.org/r/20230902061044.19366-2-tiwai@suse.de


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