usb: dwc2: gadget: Only initialize device if in device mode

In dwc2_hsotg_udc_start(), don't initialize the controller for device
mode unless we are actually in device mode.

Tested-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>