usb: gadget: mv_udc: fix kernel panic on some platforms

Some platforms will use usb to download images, the controller may not
be stopped correctly when start kernel. In some cases, it may have some
pending interrupts, and they will be triggered immediately when we finish
requesting irq in function probe. But we haven't finished the device
initialization at this time. So let's stop udc here to avoid this case
occurred.

Signed-off-by: Neil Zhang <zhangwm@marvell.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>