usb: mtu3: fix error return code in ssusb_gadget_init()

When failing to get IRQ number, platform_get_irq() may return
-EPROBE_DEFER, but we ignore it and always return -ENODEV,
so fix it.

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