net: ethernet: mediatek: Make mtk_m32 static

Fix the following sparse warning:

drivers/net/ethernet/mediatek/mtk_eth_soc.c:68:5: warning:
symbol 'mtk_m32' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Samuel Zou <zou_wei@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>