xlnx: drv: m2m: Fix SPDX license coding style

Fix SPDX license coding style. Issue is reported by checkpatch.pl as:
./scripts/checkpatch.pl --strict -f drivers/media/platform/xilinx/xilinx-m2m.c
WARNING: Missing or malformed SPDX-License-Identifier tag in line 1
 #1: FILE: drivers/media/platform/xilinx/xilinx-m2m.c:1:
+//SPDX-License-Identifier: GPL-2.0

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

drivers/media/platform/xilinx/xilinx-m2m.c

@@ -1,4 +1,4 @@
-//SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0
 /*
  * Xilinx V4L2 mem2mem driver
  *