net: phylink: fix SGMII inband autoneg enable

When we are operating in SGMII inband mode, it implies that there is a
PHY connected, and the ethtool advertisement for autoneg applies to
the PHY, not the SGMII link. When in 1000base-X mode, then this applies
to the 802.3z link and needs to be applied to the PCS.

Fix this.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1o9Ng2-005Qbe-3H@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>