ofproto: Rate-limit STP status errors on non-existent ports.

ofproto: Rate-limit STP status errors on non-existent ports.

As part of the bridge's main loop, it queries the STP status of all
ports.  If the port doesn't exist, log files can become filled with
warning messages.  This situation is very unusual, since system devices
do not normally disappear, but it's easy enough to rate-limit these
messages.

Bug #10936

Reported-by: Reid Price <reid@nicira.com>
Signed-off-by: Justin Pettit <jpettit@nicira.com>