dma: tegra: avoid channel lock up after free

Lock scenario: Channel 1 was allocated and prepared as slave_sg, used and freed.
Now preparation of cyclic dma on channel 1 will fail with err "DMA configuration
conflict" because tdc->isr_handler still setted to handle_once_dma_done.

This happens because tegra_dma_abort_all() won't be called on channel freeing
if pending list is empty and channel not busy. We need to clear isr_handler
on channel freeing to avoid locking.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/dma/tegra20-apb-dma.c

@@ -1191,6 +1191,7 @@ static void tegra_dma_free_chan_resources(struct dma_chan *dc)
 	list_splice_init(&tdc->free_dma_desc, &dma_desc_list);
 	INIT_LIST_HEAD(&tdc->cb_desc);
 	tdc->config_init = false;
+	tdc->isr_handler = NULL;
 	spin_unlock_irqrestore(&tdc->lock, flags);
 
 	while (!list_empty(&dma_desc_list)) {