ofproto: Report 0 Mbps when speed not available instead of 100 Mbps.

ofproto: Report 0 Mbps when speed not available instead of 100 Mbps.

When a link is down, or when a link has no speed because it is not a
physical interface, Open vSwitch previously reported that its rate is 100
Mbps as a default.  This is counterintuitive, however, so this commit
changes Open vSwitch behavior to report 0 Mbps when a link is down or its
speed is otherwise unavailable.

Bug #13388.
Reported-by: Hiroshi Tanaka <htanaka@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>