dmaengine: at_hdmac: fix race while monitoring channel status

We were reading channel status then taking a lock. This lead to a race because
this lock may delay us and then make this channel not idle anymore.
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dma/at_hdmac.c

@@ -853,11 +853,11 @@ static void atc_issue_pending(struct dma_chan *chan)
 
 	dev_vdbg(chan2dev(chan), "issue_pending\n");
 
-	if (!atc_chan_is_enabled(atchan)) {
 	spin_lock_bh(&atchan->lock);
+	if (!atc_chan_is_enabled(atchan)) {
 		atc_advance_work(atchan);
-		spin_unlock_bh(&atchan->lock);
 	}
+	spin_unlock_bh(&atchan->lock);
 }
 
 /**