usb: mtu3: check return value of devm_extcon_register_notifier()

Check the return value of devm_extcon_register_notifier() and
add error handling.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>