net: mtk_eth_soc: move and correct link timer programming

Program the link timer appropriately for the interface mode being
used, using the newly introduced phylink helper that provides the
nanosecond link timer interval.

The intervals are 1.6ms for SGMII based protocols and 10ms for
802.3z based protocols.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>