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>