ofw_pci: fix probing for non-DT cases

phandle_t is a uint32_t type, <= 0 comparison doesn't work with it as intended.
This caused the ofw_pci code to attach to PCI bus on ACPI based systems.
Since 3eae4e106ac7 ("Fix error value returned by ofw_bus_gen_get_node().")
ofw subsystem can only return -1 for invalid nodes. Use that.

MFC after: 4 weeks
Reviewed by: mw
Differential revision: https://reviews.freebsd.org/D30953