net: lan743x: Create separate Link Speed Duplex state function

Create separate Link Speed Duplex (LSD) update state function from
lan743x_sgmii_config () to use as subroutine.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Raju Lakkaraju <Raju.Lakkaraju@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>