libopenflow: Refactor __ne__ methods

These were all identical (just negating __eq__), so they've been factored
out into the ofp_base superclass.
libopenflow: Clean up ofp_phy_port a bit

This had some literals that should be symbolic constants, and the
_validate() wasn't entirely sensible.
of_json: Fix regression with actions

While cleaning up libopenflow actions in 4378bea837, I changed
an "internal API" which broke of_json.  Fixed.

Thanks to faizul bari for pointing it out.