The subordinate value indicates the highest bus number which can be
reached downstream though a certain device.

Commit a20c7f36 ("PCI: Do not allocate more buses than available in
parent") ensures that downstream devices cannot assign busnumbers higher
than the upstream device subordinate number, which was indeed illogical.

By default, dw_pcie_setup_rc() inits the Root Complex subordinate to a
value of 0x01.

Due to this combined with above commit, enumeration stops digging deeper
downstream as soon as bus num 0x01 has been assigned, which is always the
case for a bridge device.

This results in all devices behind a bridge bus remaining undetected, as
these would be connected to bus 0x02 or higher.

Fix this by initializing the RC to a subordinate value of 0xff, which is
not altering hardware behaviour in any way, but informs probing function
pci_scan_bridge() later on which reads this value back from register.

The following nasty errors ...