usb: dwc2: Make dwc2_force_mode() static

Declared dwc2_force_mode() function as static, because it was used
only in core.c file, for fixing sparse error.

Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>