dmaengine: amba-pl08x: 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: Dan Williams <dan.j.williams@intel.com>
Cc: Vinod Koul <vkoul@kernel.org>
Cc: dmaengine@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>