mmc: omap: Initialize dma_slave_config to avoid random data in it's fields

It is wrong to use uninitialized dma_slave_config and configure only
certain fields as the DMAengine driver might look at non initialized
(random data) fields and tries to interpret it.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>