spi: rockchip: initialize dma_slave_config properly

The rxconf and txconf structs are allocated on the
stack, so make sure we zero them before filling out
the relevant fields.

Signed-off-by: Huibin Hong <huibin.hong@rock-chips.com>
Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Signed-off-by: Mark Brown <broonie@kernel.org>