net: mediatek: fix non static symbol warnings

Fixes the following sparse warnings:

drivers/net/ethernet/mediatek/mtk_eth_soc.c:79:5: warning:
 symbol '_mtk_mdio_write' was not declared. Should it be static?
drivers/net/ethernet/mediatek/mtk_eth_soc.c:98:5: warning:
 symbol '_mtk_mdio_read' 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>