e1000e: Avoid kernel crash during shutdown

While doing shutdown on the PCI device, the corresponding callback
function e1000e_shutdown() is trying to clear those correctable
errors on the upstream P2P bridge. Unfortunately, we don't have
the upstream P2P bridge under some cases (e.g. PCI-passthrou for
KVM on Power). That leads to kernel crash eventually.

The patch adds one more check on that to avoid kernel crash.

Signed-off-by: Li Zhang <zhlcindy@gmail.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>