lan78xx: workaround of forced 100 Full/Half duplex mode error

At forced 100 Full & Half duplex mode, chip may fail to set mode correctly
when cable is switched between long(~50+m) and short one.
As workaround, set to 10 before setting to 100 at forced 100 F/H mode.

Signed-off-by: Woojung Huh <woojung.huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>