greybus: svc: keep interfaces registered during mode switch

Keep a detected interface registered until it is physically removed.
Specifically, do not re-register an interface that is switching mode.

Note that this also allows us to get rid of some nasty hacks from core.
The Ara VID/PID bootrom hack for ES2 will continue to work, but is now
mostly confined to the bootrom driver.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>