dma-buf/fence: Fix a kerneldoc warning

kerneldoc doesn't know how to parse variables, so don't let it try.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>

drivers/dma-buf/fence.c

@@ -29,7 +29,7 @@
 EXPORT_TRACEPOINT_SYMBOL(fence_annotate_wait_on);
 EXPORT_TRACEPOINT_SYMBOL(fence_emit);
 
-/**
+/*
  * fence context counter: each execution context should have its own
  * fence context, this allows checking if fences belong to the same
  * context or not. One device can have multiple separate contexts,