pcm_plugin: Fix return value of snd_pcm_rewind

pcm_plugin: Fix return value of snd_pcm_rewind

In case the rewind did not rewind as much as expected, e g due to
time delay between the latest avail update and the rewind, we must
properly account for that in the plugin layer.

Otherwise, the plugin's appl ptr and the hw's appl ptr become
unsynchronised, which is very bad, especially in mmap_shadow plugins,
e g, this could cause the overlapping memcpy in the softvol plugin
as seen here:
https://bugs.freedesktop.org/show_bug.cgi?id=64299

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>