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 <peter.ujfalusi@ti.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>