dmaengine: of_dma: Correct return code for of_dma_request_slave_channel in case !CONFIG_OF

of_dma_request_slave_channel should return either pointer for valid
dma_chan or ERR_PTR() error code, NULL is not expected to be returned.

Signed-off-by: Peter Ujfalusi <[e-mail]>
Acked-by: Arnd Bergmann <[e-mail]>
Signed-off-by: Vinod Koul <[e-mail]>

include/linux/of_dma.h

···
 static inline struct dma_chan *of_dma_request_slave_channel(struct device_node *np,
 						     const char *name)
 {
-	return NULL;
+	return ERR_PTR(-ENODEV);
 }
 
 static inline struct dma_chan *of_dma_simple_xlate(struct of_phandle_args *dma_spec,