net: phy: marvell10g: Fix Multi-G advertisement to only advertise 10G

Some Marvell Alaska PHYs support 2.5G, 5G and 10G BaseT links. Their
default behaviour is to advertise all of these modes, but at the moment,
only 10GBaseT is supported. To prevent link partners from establishing
link at that speed, clear these modes upon configuring aneg parameters.

Fixes: 20b2af32 ("net: phy: add Marvell Alaska X 88X3310 10Gigabit PHY support")
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reported-by: Russell King <linux@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>