[ALSA] echoaudio - Add barrier() to prevent compiler optimization

This patch adds a barrier() to prevent the compiler from
moving the read outside of the loop. It also fixes a comment.

Signed-off-by: Giuliano Pochini <pochini@shiny.it>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>