usb: gadget: uvc: Fix the wrong v4l2_device_unregister call

If an error occurred before calling the 'v4l2_device_register' func,
and then goto error, but no need to call 'v4l2_device_unregister'
func.

Signed-off-by: Zqiang <qiang.zhang@windriver.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>