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>

drivers/net/ethernet/mediatek/mtk_eth_soc.c

@@ -65,7 +65,7 @@ u32 mtk_r32(struct mtk_eth *eth, unsigned reg)
 	return __raw_readl(eth->base + reg);
 }
 
-u32 mtk_m32(struct mtk_eth *eth, u32 mask, u32 set, unsigned reg)
+static u32 mtk_m32(struct mtk_eth *eth, u32 mask, u32 set, unsigned reg)
 {
 	u32 val;