vswitch: Don't push bad flows to datapath during reconfiguration.

vswitch: Don't push bad flows to datapath during reconfiguration.

When reconfiguration adds a new port to a bridge, it initially has
dp_ifidx of -1, indicating that the datapath port number is unknown.  (It
can't be known because ports are added to the datapath later on.)  But
during configuration of the controller we may add or modify flows using
ofproto_add_flow(), which can update flows in the datapath.  The -1
dp_ifidx will then get converted to uint16_t as 65535, which the datapath
will reject as a bad port number.

This commit solves the problem by configuring the controller later, after
datapath port numbers have been assigned.