dt-bindings: net: dsa: b53: Fix full duplex in example

There is no such property as duplex-full. It's called full-duplex. Leading to
reduced speed when using the example as base for a real device tree.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>