dw_dmac: Mark all tx_descriptors with DMA_CRTL_ACK after xfer finish

dwc_desc_get checks all descriptors for DMA_CTRL_ACK before allocating them for
transfers. And descriptors are not marked with DMA_CRTL_ACK after transfer
finishes. Thus descriptor once used is not usable again. This patch marks
descriptors with DMA_CRTL_ACK after dma xfer finishes

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>