usb: musb: gadget: clear gadget_driver when gadget is stopped

Some musb glue drivers use gadget_driver pointer to know if any gadget
drivers are loaded at some moment and base further decisions on it,
like to do runtime suspend/resume or not. Right now the pointer is
left alone on stop and OMAP musb glue later does wrong runtime_pm
decisions because of it.

Clear the gadget_driver pointer on remove, it's invalid after stop
anyway.