usb: mtu3: remove repeated setting of gadget state

The usb_add_gadget_udc() will set the gadget state as
USB_STATE_NOTATTACHED, so we needn't set it again.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>