Fix the following gcc warning:

drivers/usb/gadget/udc/gr_udc.c:51:19: warning: ‘driver_desc’ defined
but not used [-Wunused-const-variable=]
 static const char driver_desc[] = DRIVER_DESC;
                   ^~~~~~~~~~~
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>