LinuxKPI: pci.h: always initialize return value

In pcie_capability_read_*() always initialize the return value to
avoid warnings of uninitialized values in callers.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39721

(cherry picked from commit 0e8953b94b7481a53945cbd1e16517773996e16e)