usb: dwc3: gadget: Ensure no control transfers in progress when stopping

When dwc3_gadget_pullup() is called to perform a device-initiated
disconnect, the driver must ensure that no transfers are in progress
before clearing the run/stop bit.
However, in dwc3_gadget_pullup, before the dwc->lock is acquired,
a race is possible in which a new ep0 transfer gets started and we
receive another SETUP packet from the host before proceeding to take
the lock.

To fix this, simply don't allow a new ep0 transfer to start if
dwc->softconnect is false;.

Change-Id: Ie79c99bfa2b305ef44695d01b3f9395253aaf8f8
Signed-off-by: Jack Pham <jackp@codeaurora.org>
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>