net: mtk_eth_soc: remove unused sgmii flags

The "flags" member of struct mtk_sgmii appears to be unused, as are
the MTK_SGMII_PHYSPEED_* and MTK_HAS_FLAGS() macros. Remove them.

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