dmaengine: vdma: don't crash when bad channel is requested

When client request a non existing channel from of_dma_xilinx_xlate
we get a NULL pointer dereferencing. This patch fix this problem.

Signed-off-by: Franck Jullien <franck.jullien@odyssee-systemes.fr>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>