net: pcs: xpcs: use phylink_resolve_c73() helper

Use phylink_resolve_c73() to resolve the clause 73 autonegotiation
result.

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