PCI: Fix /sys warning when sriov enabled and card is hot removed

sysfs is a bit stricter now and emits warnings in more cases.

For SRIOV hotplug, we are calling pci_stop_dev() for each VF first
(after we update pci_stop_bus_devices) which remove each VF subdir.  So
double check the VF dir in /sys before trying to remove the physfn link.

Signed-of-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>