dmaengine: dw-edma: Fix Using plain integer as NULL pointer in dw-edma-v0-debugfs.c

dmaengine: dw-edma: Fix Using plain integer as NULL pointer in dw-edma-v0-debugfs.c

Fixes warning given by executing "make C=2 drivers/dma/dw-edma/"

Sparse output:
drivers/dma/dw-edma/dw-edma-v0-debugfs.c:296:49: warning:
 Using plain integer as NULL pointer

Cc: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Link: https://lore.kernel.org/r/6569fd8ca5ddaa73afef1241ad7978c2a1fae0c7.1600206938.git.gustavo.pimentel@synopsys.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>