dt-bindings: ethernet-controller: fix fixed-link specification

The original fixed-link.txt allowed a pause property for fixed link.
This has been missed in the conversion to yaml format.

Fixes: 9d3de3c5

 ("dt-bindings: net: Add YAML schemas for the generic Ethernet options")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/E1l6W2G-0002Ga-0O@rmk-PC.armlinux.org.uk
Signed-off-by: Rob Herring <robh@kernel.org>