net: mediatek: sgmii: fix duplex configuration

The logic of the duplex bit is inverted.  Setting it means half
duplex, not full duplex.

Fix and rename macro to avoid confusion.

Fixes: 7e538372

 ("net: ethernet: mediatek: Re-add support SGMII")
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>