usb: dwc2: gadget: ignore stall check for ep0

dwc2_hsotg_start_req starts a request only if endpoint is not stalled.
Ignore this check for ep0 as core will clear DOEPCTL0.Stall after
sending stall handshake. Prepare instead for receiving next setup
packet.

Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Tested-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Tested-by: John Youn <johnyoun@synopsys.com>
Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>