net: tulip: make early_486_chipsets static

Fix the following sparse warning:

drivers/net/ethernet/dec/tulip/tulip_core.c:1280:28: warning: symbol
'early_486_chipsets' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>