dmaengine: sf-pdma: replace /** with /* for non-function comment

There are several comments starting from "/**" but not for function
comment purpose. It causes kernel-doc parsing wrong string. Replace
"/**" with "/*" to fix them.
Signed-off-by: Green Wan <green.wan@sifive.com>
Link: https://lore.kernel.org/r/20191118143554.16129-1-green.wan@sifive.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/sf-pdma/sf-pdma.c

 // SPDX-License-Identifier: GPL-2.0-or-later
-/**
+/*
  * SiFive FU540 Platform DMA driver
  * Copyright (C) 2019 SiFive
  *

drivers/dma/sf-pdma/sf-pdma.h

 /* SPDX-License-Identifier: GPL-2.0-or-later */
-/**
+/*
  * SiFive FU540 Platform DMA driver
  * Copyright (C) 2019 SiFive
  *