The Mediatek's host controller has two slots, each with its own control
registers. The host driver needs to identify what slot is connected to
what port in order to access the device's configuration space.

Current code retrieving slot connected to a given endpoint device.

Assuming each slot is connected to one endpoint device as below:

                host bridge
  bus 0 --> __________|_______
           |                  |
           |                  |
         slot 0             slot 1
  bus 1 -->|        bus 2 --> |
           |                  |
         EP 0               EP 1

During PCI enumeration, system software will scan all the PCI devices on
every bus starting from devfn 0. Using PCI_SLOT(devfn) for matching an
endpoint to its slot is erroneous in that the devfn does not contain the
hierarchical bus numbering in it. In order to match an endpoint with its
slot (and related port), the PCI tree must be walked up to the root bus
(where the root ports are situated) and then the PCI_SLOT(devfn)
matching logic can be correctly applied for matching.

This patch fixes the mtk_pcie_find_port() slot matching logic by adding
appropriate PCI tree walking code to retrieve the slot/port a given
endpoint is connected to.
Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
[lorenzo.pieralisi@arm.com: rewrote the commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>