PCI: Remove __must_check from definitions

The __must_check (gcc "warn_unused_result") attribute only makes sense
when compiling the *caller* of the function, so the attribute should
appear on the declaration in the header file, not on the definition.

The declarations of these functions are already annotated with
__must_check.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>