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: 9d0fdc602de9 ("ALSA: emu8000: Convert to generic PCM copy ops")
Reported-by: Al Viro <[e-mail]>
Closes: https://lore.kernel.org/r/20230902053646.GK3390869@ZenIV
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

sound/isa/sb/emu8000_pcm.c

···
 	/* convert to word unit */
 	pos = (pos << 1) + rec->loop_start[voice];
 	count <<= 1;
-	LOOP_WRITE(rec, pos, USER_SOCKPTR(NULL), count);
+	LOOP_WRITE(rec, pos, NULL, count);
 	return 0;
 }