usb: mtu3: sync interrupt before unbind the udc

Register gadget irq to avoid the interrupt handler is occurred
or scheduled during the unbind flow, may happen when do android
function switch stress test.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20220609074233.15532-1-chunfeng.yun@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>