bnxt_en: query cfa flow stats periodically to compute 'lastused' attribute

This patch implements periodic querying of cfa flow stats
in batches to compute the 'lastused' attribute of TC flow stats.

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>