usb: dwc2: gadget: fix gadget unregistration in udc_stop() function

udc_stop() should clear ->driver pointer unconditionally to let the UDC
framework to work correctly with both registering/unregistering gadgets
and enabling/disabling gadgets by writing to
/sys/class/udc/*hsotg/soft_connect interface.

Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>