dt-bindings: net: fec: Fix indentation

The following warning is observed when running 'make dtbs_check':
Documentation/devicetree/bindings/net/fsl,fec.yaml:85:7: [warning] wrong indentation: expected 8 but found 6 (indentation)

Fix the indentation accordingly.
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>