net: ethernet: mediatek: mark symbols static where possible

We get 2 warnings when building kernel with W=1:
drivers/net/ethernet/mediatek/mtk_eth_soc.c:2041:5: warning: no previous prototype for 'mtk_get_link_ksettings' [-Wmissing-prototypes]
drivers/net/ethernet/mediatek/mtk_eth_soc.c:2052:5: warning: no previous prototype for 'mtk_set_link_ksettings' [-Wmissing-prototypes]

In fact, these functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
So this patch marks these functions with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>