sfc: implement counters readout to TC stats

On FLOW_CLS_STATS, look up the MAE counter by TC cookie, and report the
 change in packet and byte count since the last time FLOW_CLS_STATS read
 them.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>