mmc: dw_mmc-pci: enable bus-mastering mode

This patch enables bus-mastering mode for MMC controller to allow IDMAC
transfers.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Tested-by: Prabu Thangamuthu <Prabu.T@synopsys.com>
Signed-off-by: Chris Ball <cjb@laptop.org>