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 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 during boot are also fixed by this:

  pci_bus 0000:02: busn_res: can not insert [bus 02-ff] under [bus 01] (conflicts with (null) [bus 01])
  ...
  pci_bus 0000:03: [bus 03] partially hidden behind bridge 0000:01 [bus 01]
  ...
  pci_bus 0000:04: [bus 04] partially hidden behind bridge 0000:01 [bus 01]
  ...
  pci_bus 0000:05: [bus 05] partially hidden behind bridge 0000:01 [bus 01]
  pci_bus 0000:02: busn_res: [bus 02-ff] end is updated to 05
  pci_bus 0000:02: busn_res: can not insert [bus 02-05] under [bus 01] (conflicts with (null) [bus 01])
  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 <niklas.cassel@axis.com>
Tested-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>
Cc: stable@vger.kernel.org	# v4.15+
Cc: Binghui Wang <wangbinghui@hisilicon.com>
Cc: Jesper Nilsson <jesper.nilsson@axis.com>
Cc: Jianguo Sun <sunjianguo1@huawei.com>
Cc: Jingoo Han <jingoohan1@gmail.com>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: Minghuan Lian <minghuan.Lian@freescale.com>
Cc: Mingkai Hu <mingkai.hu@freescale.com>
Cc: Murali Karicheri <m-karicheri2@ti.com>
Cc: Pratyush Anand <pratyush.anand@gmail.com>
Cc: Richard Zhu <hongxing.zhu@nxp.com>
Cc: Roy Zang <tie-fei.zang@freescale.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Stanimir Varbanov <svarbanov@mm-sol.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Xiaowei Song <songxiaowei@hisilicon.com>
Cc: Zhou Wang <wangzhou1@hisilicon.com>

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