ARM: dts: imx6dl-prtvt7: The sgtl5000 uses i2s not ac97

According to Documentation/devicetree/bindings/sound/fsl,ssi.txt
'fsl,mode' should be specified for AC97 mode only.

Signed-off-by: Robin van der Gracht <robin@protonic.nl>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>