mtk_sgmii: enable PCS polling to allow SFP work

Currently there is no IRQ handling (even the SGMII supports it).
Enable polling to support SFP ports.

Fixes: 14a44ab0

 ("net: mtk_eth_soc: partially convert to phylink_pcs")
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Alexander Couzens <lynxis@fe80.eu>
[ bmork: changed "1" => "true" ]
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>