Add missing pci_disable_device() in the error path in loongson_dwmac_probe().

Fixes: 30bba69d ("stmmac: pci: Add dwmac support for Loongson")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>