net: mtk_eth_soc: add mask and update PCS speed definitions

The PCS speed setting is a two bit field, but it is defined as two
separate bits. Add a bitfield mask for the speed definitions, an
 use the FIELD_PREP() macro to define each PCS speed.

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