iop-adma: kill debug BUG_ON

This BUG_ON caught problems in early development but now it is in the
way as it invalidly triggers when trying to remove the module.
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

drivers/dma/iop-adma.c

@@ -269,8 +269,6 @@ static void __iop_adma_slot_cleanup(struct iop_adma_chan *iop_chan)
 			break;
 	}
 
-	BUG_ON(!seen_current);
-
 	if (cookie > 0) {
 		iop_chan->completed_cookie = cookie;
 		pr_debug("\tcompleted cookie %d\n", cookie);