net: mtk_eth_soc: convert mtk_sgmii to use regmap_update_bits()

mtk_sgmii does a lot of read-modify-write operations, for which there
is a specific regmap function. Use this function instead of open-coding
the operations.

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