vswitchd: Remove port from datapath if it becomes non-operational

If kernel module rejects config changes then vswitchd sets the ofport
column to -1, but does not remove the non-operational port from the
datapath. This patch fixes this problem.

ovs-vsctl add-br ovsbr
ovs-vsctl add-port ovsbr p1
ovs-vsctl add-port ovsbr p2
ovs-vsctl set Interface p1 options:remote_ip=2.1.1.1 options:key=123 type=gre
ovs-vsctl set Interface p2 options:remote_ip=1.1.1.1 options:key=123 type=gre
ovs-vsctl set Interface p2 options:remote_ip=2.1.1.1 options:key=123 type=gre
ovs-dpctl show #observe that p2 does not appear here anymore

Signed-off-by: Ansis Atteka <aatteka@nicira.com>