PCI hotplug: fix return value of has_foo() functions

Current has_foo() functions in pci_hotplug_core.c returns 0 if the
"foo" property is true. It would cause misunderstanding. In addition,
the error code of those functions is never checked, so this patch
changes those functions' error code to 'bool' and return true if the
property "foo" is true.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Reviewed-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>