spi: tegra: slink: do not prepare dma transfer with DMA_CTRL_ACK flag

Spi starts transfer using dma with DMA_CTRL_ACK which is not require
becasue spi driver does not use completed dma_desc after transfer
done and so it does not ack the dma descriptor. Removing the
DMA_CTRL_ACK flag to avoid memory leak in dma driver.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>