usb: gadget: add _init/__exit annotations to module init/exit funcs

Add missing _init/__exit annotations to module init/exit funcs.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Link: https://lore.kernel.org/r/20220917084413.23957-1-xiujianfeng@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>