dmaengine: sh: rcar-dmac: avoid to write CHCR.TE to 1 if TCR is set to 0

This patch fixes an issue that unexpected retransfering happens
if TCR is set to 0 before rcar_dmac_sync_tcr() writes DE bit to
the CHCR register. For example, sh-sci driver can reproduce this
issue like below:

 In rx_timer_fn():		/* CHCR DE bit may be set to 1 */
  dmaengine_tx_status()
   rcar_dmac_tx_status()
    rcar_dmac_chan_get_residue()
     rcar_dmac_sync_tcr()	/* TCR is possible to be set to 0 */

According to the description of commit 73a47bd0da66 ("dmaengine:
rcar-dmac: use TCRB instead of TCR for residue"), "this buffered data
will be transferred if CHCR::DE bit was cleared". So, this patch
doesn't need to check TCRB register.

Fixes: 73a47bd0da66 ("dmaengine: rcar-dmac: use TCRB instead of TCR for residue")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/sh/rcar-dmac.c

@@ -772,8 +772,9 @@ static void rcar_dmac_sync_tcr(struct rcar_dmac_chan *chan)
 	/* make sure all remaining data was flushed */
 	rcar_dmac_chcr_de_barrier(chan);
 
-	/* back DE */
-	rcar_dmac_chan_write(chan, RCAR_DMACHCR, chcr);
+	/* back DE if remain data exists */
+	if (rcar_dmac_chan_read(chan, RCAR_DMATCR))
+		rcar_dmac_chan_write(chan, RCAR_DMACHCR, chcr);
 }
 
 static void rcar_dmac_chan_halt(struct rcar_dmac_chan *chan)