ENGR00320792-3 usb: gadget: mark init as late_initcall

Since we introduce -EPROBE_DEFER for udc driver, it will be
probed at late_initcall if it is defered. When the gadget
is built in, it will return "couldn't find an available UDC"
at such case. That's the problem we met at below link:

http://marc.info/?l=linux-usb&m=137706435611447&w=2



We have no driver's probe at gadget driver, so we can't return
-EPROBE_DEFER. And it is also not suitable to defer udc_bind_to_driver
if the udc is not found temporarily, since it is hard to decide the
return value for usb_gadget_probe_driver.

Due to above reasons, mark gadget's init as late_initcall may be a
moderate solution.

Signed-off-by: Peter Chen <peter.chen@freescale.com>