net: phylink: use a dedicated helper to parse usgmii control word

Q-USGMII is a derivative of USGMII, that uses a specific formatting for
the control word. The layout is close to the USXGMII control word, but
doesn't support speeds over 1Gbps. Use a dedicated decoding logic for
the USGMII control word, re-using USXGMII definitions but only considering
10/100/1000Mbps speeds

Fixes: 5e61fe15 ("net: phy: Introduce QUSGMII PHY mode")
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>