usb: dwc2: Fix NULL qh in dwc2_queue_transaction

When a usb device disconnects in a certain way, dwc2_queue_transaction
still gets called after dwc2_hcd_cleanup_channels.

dwc2_hcd_cleanup_channels does "channel->qh = NULL;" but
dwc2_queue_transaction still wants to dereference qh.
This adds a check for a null qh.

Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Alexandru M Stan <amstan@chromium.org>
[dianders: rebased to mainline]
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>