net: aquantia: make function aq_fw2x_update_stats static

Fixes the following sparse warning:

 drivers/net/ethernet/aquantia/atlantic/hw_atl/hw_atl_utils_fw2x.c:282:5: warning:
  symbol 'aq_fw2x_update_stats' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>