device/pciexp_device.c: Fix setting Max Payload Size

Current implementation assumes that the endpoint device is connected
directly to the PCIe Root Port, which does not always have to be true.
In a case where there is a PCIe switch between the endpoint and the
root port, the Max Payload Size capability may differ across the
devices in the chain and coreboot will not set a correct Max Payload
Size. This results in a PCIe device malfunction in pre-OS environment,
e.g. if the Ethernet NICs are connected behind a PCIe switch, the iPXE
fails to obtain the DHCP configuration.

Fix this by traversing the topology and programming the highest common
Max Payload Size in the given PCIe device chain during enumeration.
Once finished, the root port has the highest common Max Payload Size
supported by all the devices in the chain. So at the end of root port
bus scan, propagate the root port's Max Payload Size to all downstream
devices to keep Max Payload Size in sync within the whole chain.

TEST=Perform successful dhcp command in iPXE on the NIC connected to
the PCIe root port via ASMedia ASM1806 PCIe switch and again on the
NIC connected directly to the PCIe root port.

Change-Id: I24386dc208363b7d94fea46dec25c231a3968225
Signed-off-by: Michał Żygowski <michal.zygowski@3mdeb.com>
---
M src/device/pciexp_device.c
1 file changed, 129 insertions(+), 28 deletions(-)