bnxt_en: add code to query TC flower offload stats

This patch adds code to implement TC_CLSFLOWER_STATS TC-cmd and the
required FW code to query the stats from the HW.

Signed-off-by: Sathya Perla <sathya.perla@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>