dmaengine: kill some dubious WARN_ONCEs

dma_find_channel and dma_issue_pending_all are good places to warn about
improper api usage.  However, warning correctly means synchronizing with
dma_list_mutex, i.e. too much overhead for these fast-path calls.
Reported-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

drivers/dma/dmaengine.c

@@ -329,9 +329,6 @@ struct dma_chan *dma_find_channel(enum dma_transaction_type tx_type)
 	struct dma_chan *chan;
 	int cpu;
 
-	WARN_ONCE(dmaengine_ref_count == 0,
-		  "client called %s without a reference", __func__);
-
 	cpu = get_cpu();
 	chan = per_cpu_ptr(channel_table[tx_type], cpu)->chan;
 	put_cpu();
@@ -348,9 +345,6 @@ void dma_issue_pending_all(void)
 	struct dma_device *device;
 	struct dma_chan *chan;
 
-	WARN_ONCE(dmaengine_ref_count == 0,
-		  "client called %s without a reference", __func__);
-
 	rcu_read_lock();
 	list_for_each_entry_rcu(device, &dma_device_list, global_node) {
 		if (dma_has_cap(DMA_PRIVATE, device->cap_mask))