ASoC: Name iov_iter argument as iterator instead of buffer

ASoC: Name iov_iter argument as iterator instead of buffer

While transitioning ASoC code for iov_iter usages, I kept the argument
name as "buf" as the original code.  But, iov_iter is an iterator, and
using the name "buf" may be misleading: the crucial difference is that
iov_iter can be proceeded after the operation, hence it can't be
passed twice, while a simple "buffer" sounds as if reusable.

To make the usage clearer, rename the argument from "buf" to "iter".
There is no functional changes, just names.

Fixes: 66201cacc33d ("ASoC: component: Add generic PCM copy ops")
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/CAHk-=wje+VkXjjfVTmK-uJdG_M5=ar14QxAwK+XDiq07k_pzBg@mail.gmail.com
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230831130457.8180-2-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>