net: mtk_eth_soc: add out of band forcing of speed and duplex in pcs_link_up

Add support for forcing the link speed and duplex setting in the
pcs_link_up() method for out of band modes, which will be useful when
we finish converting the pcs_config() method. Until then, we still have
to force duplex for 802.3z modes to work correctly.

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