Create specific types for ofp and odp port

Create specific types for ofp and odp port

Until now, datapath ports and openflow ports were both represented by
unsigned integers of various sizes. With implicit conversions, etc., it is
easy to mix them up and use one where the other is expected.  This commit
creates two typedefs, ofp_port_t and odp_port_t.  Both of these two types
are marked by "__attribute__((bitwise))" so that sparse can be used to
detect any misuse.

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