dmaengine: coh901318: no need to cast away call to debugfs_create_file()

No need to check the return value of debugfs_create_file(), so no need
to provide a fake "cast away" of the return value either.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Vinod Koul <vkoul@kernel.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: dmaengine@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>