sfc: fix sparse non static symbol warning

Fixes the following sparse warning:

drivers/net/ethernet/sfc/falcon.c:2601:6: warning:
 symbol 'falcon_pull_nic_stats' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>