usb: dwc3: gadget: Synchronize IRQ between soft connect/disconnect

Ensure that there are no pending events being handled in between soft
connect/disconnect transitions.  As we are keeping interrupts enabled,
and EP0 events are still being serviced, this avoids any stale events from
being serviced.

Reviewed-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>
Link: https://lore.kernel.org/r/20220817182359.13550-4-quic_wcheng@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>