ata: sata_dwc_460ex: Remove unused forward declaration

sata_dwc_port_stop() is not used before being defined,
remove redundant forward declaration.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>