net: hns: make function hns_gmac_wait_fifo_clean() static

Fixes the following sparse warning:

drivers/net/ethernet/hisilicon/hns/hns_dsaf_gmac.c:322:5: warning:
 symbol 'hns_gmac_wait_fifo_clean' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>