dmaengine: dw-edma: Add dw_edma prefix to the DebugFS nodes descriptor

The rest of the locally defined and used methods and structures have
dw_edma prefix in their names. It's right in accordance with the kernel
coding style to follow the locally defined rule of naming. Let's add that
prefix to the debugfs_entries structure too especially seeing it's name
may be confusing as if that structure belongs to the global DebugFS space.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Tested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-By: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>