usb: dwc3: gadget: Queue PM runtime idle on disconnect event

There is a scenario where DWC3 runtime suspend is blocked due to the
dwc->connected flag still being true while PM usage_count is zero after
DWC3 giveback is completed and the USB gadget session is being terminated.
This leads to a case where nothing schedules a PM runtime idle for the
device.

The exact condition is seen with the following sequence:
  1.  USB bus reset is issued by the host
  2.  Shortly after, or concurrently, a USB PD DR SWAP request is received
      (sink->source)
  3.  USB bus reset event handler runs and issues
      dwc3_stop_active_transfers(), and pending transfer are stopped
  4.  DWC3 usage_count decremented to 0, and runtime idle occurs while
      dwc->connected == true, returns -EBUSY
  5.  DWC3 disconnect event seen, dwc->connected set to false due to DR
      swap handling
  6.  No runtime idle after this point

Address this by issuing an asynchronous PM runtime idle call after the
disconnect event is completed, as it modifies the dwc->connected flag,
which is what blocks the initial runtime idle.

Fixes: fc8bb91b

 ("usb: dwc3: implement runtime PM")
Cc:  <stable@vger.kernel.org>
Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>
Link: https://lore.kernel.org/r/20240103214946.2596-1-quic_wcheng@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>