dmaengine: at_hdmac: no need set ACK in new descriptor

Following descriptor flow in at_hdmac driver, descriptor comming from
atc_desc_get() as already DMA_CTRL_ACK flag set. No need to set it again.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>