ci_udc_unregister() used to unregister "the controller". Since we
may register multiple chipidea devices we called ci_udc_unregister()
for each of them. This led to messages like:

ERROR: imx-usb 53f80000.usb: gadget not registered.

Fix this by returning the registered controller. This allows us to call
ci_udc_unregister() only when we actually registered one before.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>