lan743x: Use correct MAC_CR configuration for 1 GBit speed

lan743x: Use correct MAC_CR configuration for 1 GBit speed

Corrected the MAC_CR configuration bits for 1 GBit operation. The data
sheet allows MAC_CR(2:1) to be 10 and also 11 for 1 GBit/s speed, but
only 10 works correctly.

Devices tested:
Microchip Lan7431, fixed-phy mode
Microchip Lan7430, normal phy mode

Fixes: e625dae96783 ("lan743x: Added fixed link and RGMII support")
Signed-off-by: Roelof Berg <rberg@berg-solutions.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>