net: phylink: report correct max speed for QUSGMII

Q-USGMII is the quad port version of USGMII, and supports a max speed of
1Gbps on each line. Make so that phylink_interface_max_speed() reports
this information correctly.

Fixes: ae0e4bb2 ("net: phylink: Adjust link settings based on rate matching")
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>