Add 3 counters per priority to ethtool using PPCNT:
1) rx_prio[p]_buf_discard - the number of packets discarded by device
due to lack of per host receive buffers
2) rx_prio[p]_cong_discard - the number of packets discarded by device
due to per host congestion
3) rx_prio[p]_marked - the number of packets ECN marked by device due
to per host congestion