staging: mt7621-dma: Remove assignment in if

Fixes checkpatch error: ASSIGN_IN_IF by adding an inner if in the else
path, this also avoids calling vchan_find_desc when not needed.
Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-dma/ralink-gdma.c

@@ -689,8 +689,11 @@ static enum dma_status gdma_dma_tx_status(struct dma_chan *c,
 				((chan->next_sg - 1) * desc->sg[0].len);
 		else
 			state->residue = desc->residue;
-	} else if ((vdesc = vchan_find_desc(&chan->vchan, cookie)))
-		state->residue = to_gdma_dma_desc(vdesc)->residue;
+	} else {
+		vdesc = vchan_find_desc(&chan->vchan, cookie);
+		if (vdesc)
+			state->residue = to_gdma_dma_desc(vdesc)->residue;
+	}
 	spin_unlock_irqrestore(&chan->vchan.lock, flags);
 
 	dev_dbg(c->device->dev, "tx residue %d bytes\n", state->residue);