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>