spi: rockchip: simplify use_dma logic

We only need to know if we're using dma when setting
up the transfer, so just use a local variable for
that.

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