usb: cdnsp: Mark cdnsp_gadget_ops with static keyword

Fix the following sparse warning:

drivers/usb/cdns3/cdnsp-gadget.c:1546:29: warning: symbol 'cdnsp_gadget_ops' was not declared. Should it be static?

Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>