spi: rockchip: don't store dma channels twice

The spi master (aka spi controller) structure already
has two fields for storing the rx and tx dma channels.
Just use them rather than duplicating them in driver data.

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