usb: dwc2: do not override forced dr_mode in gadget setup

The host/device mode set with dr_mode should be kept all the time,
not being changed to OTG in gadget setup (by overriding CFGUSB_FORCEDEVMODE
and CFGUSB_FORCEHOSTMODE bits).

Acked-by: John Youn <johnyoun@synopsys.com>
Tested-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Przemek Rudy <prudy1@o2.pl>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>