spi: rockchip: initialize dma_slave_config properly

[ Upstream commit dd8fd2cb ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Change-Id: Ib9a7d4c869e750ced961a62feab2a59c84f06801