dmaengine: xilinx_dpdma: use correct SDPX tag for header file

dmaengine: xilinx_dpdma: use correct SDPX tag for header file

Commit 188c310bdd5d ("dmaengine: xilinx_dpdma: stop using slave_id
field") add the header file with incorrect format for SPDX tag, fix that

WARNING: Improper SPDX comment style for 'include/linux/dma/xilinx_dpdma.h', please use '/*' instead
#1: FILE: include/linux/dma/xilinx_dpdma.h:1:
+// SPDX-License-Identifier: GPL-2.0

WARNING: Missing or malformed SPDX-License-Identifier tag in line 1
#1: FILE: include/linux/dma/xilinx_dpdma.h:1:
+// SPDX-License-Identifier: GPL-2.0

Fixes: 188c310bdd5d ("dmaengine: xilinx_dpdma: stop using slave_id field")
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Link: https://lore.kernel.org/r/20211213052141.850807-1-vkoul@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>