vswitchd: Reset ofport columns of invalid interfaces to -1.

vswitchd: Reset ofport columns of invalid interfaces to -1.

Until now, when ovs-vswitchd dropped interfaces because of duplicate names,
it would not write -1 to the ofport column in the OVSDB record for the
interface, which made it appear that the interface was still active (if
it had been before).  This commit fixes the problem.

This commit also sets ofport to -1 for interfaces that cannot be created
for other reasons.