ofproto-dpif-xlate: Fix confusion between "no stp port" and "stp port 0".

ofproto-dpif-xlate: Fix confusion between "no stp port" and "stp port 0".

Commit 9d189a50e (ofproto-dpif-xlate: Pull STP xlation into
ofproto-dpif-xlate.) introduced the bug that considers 'stp_port_no'
of 0 as stp disabled on the port.  However 'stp_port_no' is
actually the index of the stp struct's port array and ranges
between [0, STP_MAX_PORTS).  So the bug allows the blocked
port keep transmitting packets and generates loop.

This commit fixes this bug.

Signed-off-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>