net: dsa: mv88e6xxx: Respect SPEED_UNFORCED, don't set force bit

The SPEED_UNFORCED indicates the MAC & PHY should perform
auto-negotiation to determine a speed which works. If this is called
for, don't set the force bit. If it is set, the MAC actually does
10Gbps, why the internal PHYs don't support.

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