[pci] Ignore invalid subordinate bus numbers

[pci] Ignore invalid subordinate bus numbers

Some systems (observed on a Dell C6615) fail to correctly populate the
subordinate PCI bus number on some PCI bridges.  We do not currently
guard against this behaviour, causing us to subsequently scan through
a huge expanse of the PCI bus:dev.fn address range.

Fix by ignoring the subordinate bus number if it is lower than the
bridge's own bus number.

Reported-by: Anisse Astier <an.astier@criteo.com>
Reported-by: Ahmad Mahagna <ahmhad@nvidia.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>