PCI: dwc: Fix enumeration end when reaching root subordinate

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

Commit a20c7f36bd3d ("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 to remain 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.

Following nasty errors during boot are also fixed by this:

[    0.459145] pci_bus 0000:02: busn_res: can not insert [bus 02-ff]
under [bus 01] (conflicts with (null) [bus 01])
...
[    0.464515] pci_bus 0000:03: [bus 03] partially hidden behind bridge
0000:01 [bus 01]
...
[    0.464892] pci_bus 0000:04: [bus 04] partially hidden behind bridge
0000:01 [bus 01]
...
[    0.466488] pci_bus 0000:05: [bus 05] partially hidden behind bridge
0000:01 [bus 01]
[    0.466506] pci_bus 0000:02: busn_res: [bus 02-ff] end is updated to
05
[    0.466517] pci_bus 0000:02: busn_res: can not insert [bus 02-05]
under [bus 01] (conflicts with (null) [bus 01])
[    0.466534] pci_bus 0000:02: [bus 02-05] partially hidden behind
bridge 0000:01 [bus 01]

Fixes: a20c7f36bd3d ("PCI: Do not allocate more buses than available in
parent")
Tested-by: Niklas Cassel <[email protected]>
Tested-by: Fabio Estevam <[email protected]>
Tested-by: Sebastian Reichel <[email protected]>
Signed-off-by: Koen Vandeputte <[email protected]>
Signed-off-by: Lorenzo Pieralisi <[email protected]>
Reviewed-by: Mika Westerberg <[email protected]>
Acked-by: Lucas Stach <[email protected]>
Cc: <[email protected]> # 4.15
Cc: Binghui Wang <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Cc: Jesper Nilsson <[email protected]>
Cc: Jianguo Sun <[email protected]>
Cc: Jingoo Han <[email protected]>
Cc: Kishon Vijay Abraham I <[email protected]>
Cc: Lorenzo Pieralisi <[email protected]>
Cc: Lucas Stach <[email protected]>
Cc: Mika Westerberg <[email protected]>
Cc: Minghuan Lian <[email protected]>
Cc: Mingkai Hu <[email protected]>
Cc: Murali Karicheri <[email protected]>
Cc: Pratyush Anand <[email protected]>
Cc: Richard Zhu <[email protected]>
Cc: Roy Zang <[email protected]>
Cc: Shawn Guo <[email protected]>
Cc: Stanimir Varbanov <[email protected]>
Cc: Thomas Petazzoni <[email protected]>
Cc: Xiaowei Song <[email protected]>
Cc: Zhou Wang <[email protected]>

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