PCI: Align PCIe capability and PCI accessor return values

The PCI config accessors (pci_read_config_word(), et al) return
PCIBIOS_SUCCESSFUL (zero) or positive error values like
PCIBIOS_FUNC_NOT_SUPPORTED.

The PCIe capability accessors similarly return PCIBIOS errors, but in
addition, they can return -EINVAL.  This makes it harder than it should be
to check for errors.

Return PCIBIOS_BAD_REGISTER_NUMBER instead of -EINVAL in all PCIe
capability accessors.

Suggested-by: Bjorn Helgaas <bjorn@helgaas.com>
Link: https://lore.kernel.org/r/20200615073225.24061-9-refactormyself@gmail.com


Signed-off-by: Bolarinwa Olayemi Saheed <refactormyself@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>