Trigger callback is not the best place for buffer iterator
initialization, so move it out to the prepare callback, where it
have to be.

Minor enhancement: remove blank line.
Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Link: https://lore.kernel.org/r/20230804110740.9867-1-ivan.orlov0322@gmail.comSigned-off-by: Takashi Iwai <tiwai@suse.de>