net: pcs: xpcs: use mii_c73_to_linkmode() helper

Convert xpcs clause 73 reading to use the newly introduced
mii_c73_to_linkmode() helper to translate the link partner
advertisement to an ethtool bitmap.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>