bridge: Consistently use miimon status if miimon is configured.

bridge: Consistently use miimon status if miimon is configured.

A port can be configured to use miimon reporting as the criterion for
enabling or disabling an interface, but in some cases (such as for reading
the initial link status) the code was reading the carrier status instead.
This commit fixes the problem.

This changes the meaning of the link_status column in the Interface table.
I don't think that the old meaning was useful to the controller in the
case of a bond configured for miimon monitoring, because the controller
could not use it to detect which interfaces the bond considered to be up
or down.