datapath: Get rid of ODPP_NONE.

This definition wasn't actually useful for the kernel--the only place that
it was used it didn't really have to be, so this commit removes it from
datapath-protocol.h.  It is still marginally useful in userspace, at least
as a value that converts to and from OpenFlow port number OFPP_NONE, so
move it to odp-util.c.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>