usb: dwc2: gadget: fix clear halt feature handling

When clearing HALT on an endpoint, req->complete of in progress
requests must be called with locks off. New request should only be
started if there is not already a pending request on the endpoint.

Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>