spi: s3c64xx: Zero dma_slave_config struct in prepare_dma()

Not all fields of dma_slave_config struct are being initialized by
prepare_dma() function, leaving those which are not in undefined state,
which can confuse DMA drivers using them.

This patch adds call to memset() to zero the struct before initializing
a subset of its fields.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>