ALSA: pcmtest: Move buffer iterator initialization to prepare callback

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.com


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