usb: dwc3: gadget: set the OTG flag in dwc3 gadget driver.

This patch is needed in order to pass one test case
defined in the OTG Automated Compliance Test specification.

Specification location:
http://www.usb.org/developers/onthego/otgeh_compliance_plan_1_2.pdf

This test case uses PET Tool, and PET Tool is one USB hardware
equipment provided by MQP Electronics.

Test case id is 6.8.3 B-UUT Bypass Capacitance.

We must set this otg flag in order to be able to return OTG
descriptor during enumeration, otherwise this test case with
failed with below error: "Get OTG descriptor request stalled".
Signed-off-by: Jianqiang Tang <jianqiang.tang@intel.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>

drivers/usb/dwc3/gadget.c

@@ -2789,6 +2789,7 @@ int dwc3_gadget_init(struct dwc3 *dwc)
 	dwc->gadget.speed		= USB_SPEED_UNKNOWN;
 	dwc->gadget.sg_supported	= true;
 	dwc->gadget.name		= "dwc3-gadget";
+	dwc->gadget.is_otg		= dwc->dr_mode == USB_DR_MODE_OTG;
 
 	/*
 	 * FIXME We might be setting max_speed to <SUPER, however versions