usb: chipidea: Drop lock across event_notify during gadget stop

The CI_HDRC_CONTROLLER_STOPPED_EVENT may want to call sleeping
APIs similar to how _gadget_stop_activity() may. Let's drop the
lock across the event so that glue drivers can make sleeping
calls.

Cc: Peter Chen <peter.chen@nxp.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stephen Boyd <stephen.boyd@linaro.org>
Signed-off-by: Peter Chen <peter.chen@nxp.com>