ARM: dts: imx6q-tbs2910: Remove unneeded 'fsl,mode' property

imx6q-tbs2910 board uses 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 this property.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Tested-by: Soeren Moch <smoch@web.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>