ARM: dts: imx6qdl-ts7970: Drop redundant "fsl,mode" option

The operating mode is used for the AC97 interface only,
so lets drop the excess fsl,mode item from SSI node.
Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6qdl-ts7970.dtsi

@@ -506,7 +506,6 @@ &snvs_rtc {
 };
 
 &ssi1 {
-	fsl,mode = "i2s-slave";
 	status = "okay";
 };