openvswitch: avoid cast-qual warning in vport_priv

This function must cast a const value to a non const value.
By adding an uintptr_t cast the warning is suppressed.
To avoid the cast (proper solution) several function signatures
must be changed.

Signed-off-by: Daniele Di Proietto <daniele.di.proietto@gmail.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>