spi: rockchip: always use SPI mode

The hardware supports 3 different variants of SPI
and there were some code around it, but nothing
to actually set it to anything but "Motorola SPI".
Just drop that code and always use that mode.

Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Mark Brown <broonie@kernel.org>