net: ethernet: mtk_eth_soc: only write values if needed

Only restart auto-negotiation and write link timer if actually
necessary. This prevents losing the link in case of minor
changes.

Fixes: 7e538372 ("net: ethernet: mediatek: Re-add support SGMII")
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: David S. Miller <davem@davemloft.net>