pseudoport: fix arbitrary instance for physical ports

Physical ports have a maximum that is lower than the maximum value of
uint16, specifically OFPP_MAX in the OpenFlow 1.0 standard.

Related to 8f66363b45b160e8b59f4ad8f4e20215d4161376.
Merge pull request #142 from frenetic-lang/pseudoport

fix arbitrary instance for physical ports