dmaengine: at_xdmac: double FIFO flush needed to compute residue

	residue = desc->xfer_size;

	/*

	 * Flush FIFO: only relevant when the transfer is source peripheral

	 * synchronized.

	 * synchronized. Flush is needed before reading CUBC because data in

	 * the FIFO are not reported by CUBC. Reporting a residue of the

	 * transfer length while we have data in FIFO can cause issue.

	 * Usecase: atmel USART has a timeout which means I have received

	 * characters but there is no more character received for a while. On

	 * timeout, it requests the residue. If the data are in the DMA FIFO,

	 * we will return a residue of the transfer length. It means no data

	 * received. If an application is waiting for these data, it will hang

	 * since we won't have another USART timeout without receiving new

	 * data.

	 */

	mask = AT_XDMAC_CC_TYPE | AT_XDMAC_CC_DSYNC;

	value = AT_XDMAC_CC_TYPE_PER_TRAN | AT_XDMAC_CC_DSYNC_PER2MEM;

		goto spin_unlock;

	}

	/*

	 * Flush FIFO: only relevant when the transfer is source peripheral

	 * synchronized. Another flush is needed here because CUBC is updated

	 * when the controller sends the data write command. It can lead to

	 * report data that are not written in the memory or the device. The

	 * FIFO flush ensures that data are really written.

	 */

	if ((desc->lld.mbr_cfg & mask) == value) {

		at_xdmac_write(atxdmac, AT_XDMAC_GSWF, atchan->mask);

		while (!(at_xdmac_chan_read(atchan, AT_XDMAC_CIS) & AT_XDMAC_CIS_FIS))

			cpu_relax();

	}

	/*

	 * Remove size of all microblocks already transferred and the current

	 * one. Then add the remaining size to transfer of the current