wwan: check new uci bus option on proto setup event

wwan: check new uci bus option on proto setup event

If system has more then one and different wwan interface (modem). Then the
wwan protohandler will always take the modem which is discovered first.
The protohandler will always setup the same interface. To fix this add a
new usb "bus" option which is associated with wwan device and so will set
the specified interface up. With this change more then one interface
could be mananged by the wwan protohandler.

If the "bus" option is not set in the uci network config then the protohandler
behaves as before the change. The protohanldler will take the first
interface which he founds.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>