LinuxKPI: implement pcie_capability_set_word()

Implement pcie_capability_set_word() using the already available
read/write functions.
Also define the completion timeout disable value to our PCI one.
Both needed by a driver update.

Sponsored by: The FreeBSD Foundation
Reviewed by: manu, hselasky
Differential Revision: https://reviews.freebsd.org/D35250

(cherry picked from commit 85eb99f9a47151f90565a1fb37395b8302c2978c)