USB mode white-list configuration can get applied via D-Bus during bootup.
This requires re-evaluation of the currently active usb mode - which
misfires when nothing is connected to usb port and usb mode gets changed
from undefined to charging fallback (= charging from pc).

Add explicit check for undefined usb mode.
Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>