usb: cdns3: gadget: own the lock wrongly at the suspend routine

When the system goes to suspend, if the controller is at device mode with
cable connecting to host, the call stack is: cdns3_suspend->
cdns3_gadget_suspend -> cdns3_disconnect_gadget, after cdns3_disconnect_gadget
is called, it owns lock wrongly, it causes the system being deadlock after
resume due to at cdns3_device_thread_irq_handler, it tries to get the lock,
but can't get it forever.

To fix it, we delete the unlock-lock operations at cdns3_disconnect_gadget,
and do it at the caller.

Fixes: b1234e3b ("usb: cdns3: add runtime PM support")
Acked-by: Pawel Laszczak <pawell@cadence.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>