flow_format: Do not format "in_port=0".

flow_format: Do not format "in_port=0".

flow_format() is used mainly for formating the headers of packets
embedded in OpenFlow PACKET_IN messages.  In this case the flow does
not have a valid port number, and printing out "in_port=0" only
confuses the resulting output.  Besides, 0 is not a valid OpenFlow
port number.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>