ata: sata_dwc_460ex: DMA is always a flow controller

In the original code the DMA is always a flow controller. Set this accordingly
in updated code.

Tested-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>