usb: musb: gadget: beautify usb_gadget_probe_driver()/usb_gadget_unregister_driver

Just a few cosmetic fixes to usb_gadget_probe_driver()
and usb_gadget_unregister_driver().

Decreased a few indentation levels with goto statements.

While at that, also add the missing call to musb_stop().
If we don't have OTG, there's no point of leaving
MUSB prepared for operation if a gadget driver fails
to probe. The same is valid for usb_gadget_unregister_driver(),
since we are removing the gadget driver and we don't have
OTG, we can completely unconfigure MUSB.

Signed-off-by: Felipe Balbi <balbi@ti.com>