The gadget API function usb_ep_set_halt() expects the gadget to return
-EAGAIN if the ep is active. Add support for this behavior.

Otherwise this may break mass storage protocol if a STALL is attempted
on the endpoint.
Signed-off-by: NVahram Aharonyan <vahrama@synopsys.com>
Signed-off-by: NJohn Youn <johnyoun@synopsys.com>
Signed-off-by: NFelipe Balbi <felipe.balbi@linux.intel.com>