usb: mtu3: fix error code for getting extcon device

When failing to get extcon device, extcon_get_edev_by_phandle()
may return different error codes, but not only -EPROBE_DEFER,
so can't always return -EPROBE_DEFER, and fix it.

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