net: phy: ar803x: disable extended next page bit

This bit is enabled by default and advertises support for extended
next page support.  XNP is only needed for 10GBase-T and MultiGig
support which is not supported. Additionally, Cisco MultiGig switches
will read this bit and attempt 10Gb negotiation even though Next Page
support is disabled. This will cause timeouts when the interface is
forced to 100Mbps and auto-negotiation will fail. The interfaces are
only 1000Base-T and supporting auto-negotiation for this only requires
the Next Page bit to be set.

Taken from:
https://github.com/SolidRun/linux-stable/commit/7406c5244b7ea6bc17a2afe8568277a8c4b126a9
and adapted to mainline kernels by rmk.
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/E1kzSdb-000417-FJ@rmk-PC.armlinux.org.ukSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/at803x.c

@@ -587,7 +587,13 @@ static int at803x_config_init(struct phy_device *phydev)
 			return ret;
 	}
 
-	return 0;
+	/* Ar803x extended next page bit is enabled by default. Cisco
+	 * multigig switches read this bit and attempt to negotiate 10Gbps
+	 * rates even if the next page bit is disabled. This is incorrect
+	 * behaviour but we still need to accommodate it. XNP is only needed
+	 * for 10Gbps support, so disable XNP.
+	 */
+	return phy_modify(phydev, MII_ADVERTISE, MDIO_AN_CTRL1_XNP, 0);
 }
 
 static int at803x_ack_interrupt(struct phy_device *phydev)