PCI/P2PDMA: Whitelist Intel Skylake-E Root Ports at any devfn

In 7b94b53d ("PCI/P2PDMA: Add Intel Sky Lake-E Root Ports B, C, D to
the whitelist"), Andrew Maier added Skylake-E 2031, 2032, and 2033 Root
Ports to the pci_p2pdma_whitelist[], so we assume P2PDMA between devices
below these ports works.

Previously we only checked the whitelist for a device at devfn 00.0 on the
root bus, which is often a "host bridge".  But these Skylake Root Ports may
be at any devfn and there may be no "host bridge" device.

Generalize pci_host_bridge_dev() so we check the first device on the root
bus, whether it is devfn 00.0 or a PCIe Root Port, against the whitelist.

[bhelgaas: commit log, comment]
Link: https://lore.kernel.org/r/20220410105213.690-2-shlomop@pliops.com


Tested-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Shlomo Pongratz <shlomop@pliops.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Andrew Maier <andrew.maier@eideticom.com>