PCI: fix hotplug get_##name return value problem

Currently, get_##name in pci_hotplug_core.c will return 0 if module
unload wins the race between unload & reading the hotplug file.  Fix
that case to return -ENODEV like it should.

Reviewed-by: Alex Chiang <achiang@hp.com>
Reviewed-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Yu Zhao <yu.zhao@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>