ARM: dts: hummingboard: Remove unneeded 'fsl,mode' property

imx6qdl-hummingboard boards use sgtl5000 codec and the machine file
(imx-sgtl5000) already sets SSI in slave mode and codec in master mode, so there
is no need for having the 'fsl,mode' property.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>