dpif: Abstract translation from OpenFlow queue ID into ODP priority value.

dpif: Abstract translation from OpenFlow queue ID into ODP priority value.

When the QoS code was integrated, I didn't yet know how to abstract the
translation from a queue ID in an OpenFlow OFPAT_ENQUEUE action into a
priority value for an ODP ODPAT_SET_PRIORITY action.  This commit is a
first attempt that works OK for Linux, so far.  It's possible that in fact
this translation needs the 'netdev' as an argument too, but it's not needed
yet.