The packets priority queue scheduling mechanism. There are 3 different
modes: 'Strict priority(SP)', 'Weighted fair queue(WFR)' and 'Mixed SP & WFR'.
SP: Packets residing in the higher priority queues transmit first.
Lower priority packets transmit once these queues are emptied.
Highest priority Q3 goes first always, follows Q2 if Q3 is empty,
then Q1 and lowest Q0.
WFR:Each queue is assigned a service weight ratio that determines how many
packets are sent from it. Weight is fixed at 8:4:2:1 for Q3:Q2:Q1:Q0.
MIX:Highest priority Q3 use SP, and other queues conform to WFR at 4:2:1 weight.