net: ethernet: mtk_eth_soc: fix register definitions for MT7988

More register macros need to be adjusted for the 3rd GMAC on MT7988.
Account for added bit in SYSCFG0_SGMII_MASK.

Fixes: 445eb644 ("net: ethernet: mtk_eth_soc: add basic support for MT7988 SoC")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/1c8da012e2ca80939906d85f314138c552139f0f.1692721443.git.daniel@makrotopia.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>