spi: rockchip: use designated init for dma config

Use C99 designated initializers for dma slave config
structures. This also makes sure uninitialized fields
are zeroed so we don't need an explicit memset.

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