x86/pci: Rename 'MMCONFIG' to 'ECAM', use pr_fmt

The "MMCONFIG" term is not used in PCI/PCIe specs.  Replace it with "ECAM",
the term used in PCIe r6.0, sec 7.2.2.

Define pr_fmt() instead of repeating PREFIX in every log message.

Link: https://lore.kernel.org/r/20231121183643.249006-5-helgaas@kernel.org


Tested-by: Tomasz Pala <gotar@polanet.pl>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>