sink/source: Initialize port before fixate hook (fixes volume/mute not saved)
In case a port has not yet been saved, which is e g often the case
if a sink/source has only one port, reading volume/mute will be done
without port, whereas writing volume/mute will be done with port.
Work around this by setting a default port before the fixate hook,
so module-device-restore can read volume/mute for the correct port.
BugLink: https://bugs.launchpad.net/bugs/1289515
Signed-off-by: David Henningsson <david.henningsson@canonical.com>