dma: of: Fix of_node reference leak

of_dma_request_slave_channel() currently does not drop the reference to the
dma_spec of_node if no DMA controller matching the of_node could be found. This
patch fixes it by always calling of_node_put().

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>