libusbg: Fix usbg_disable_gadget to actually clear the UDC

Currently usbg_disable_gadget() does not actually write anything
to UDC to clear it and the configured UDC name stays there.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Krzysztof Opasiak <k.opasiak@samsung.com>