"pci_mmcfg_check_reserved()" doesn't give a hint about what the boolean
return value means.  Rename it to pci_mmcfg_reserved() so testing
"if (pci_mmcfg_reserved())" makes sense.

Update callers to treat the return value as boolean instead of comparing
with 0.

Link: https://lore.kernel.org/r/20231121183643.249006-7-helgaas@kernel.orgTested-by: Tomasz Pala <gotar@polanet.pl>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>