LinuxKPI: Add pcie_capability_clear_and_set_word() function

It does a Read-Modify-Write operation using clear and set bitmasks on
PCI Express Capability Register at pos. As certain PCI Express
Capability Registers are accessed concurrently in RMW fashion, hence
require locking which is handled transparently to the caller.

Sponsored by: Serenity CyberSecurity, LLC
Reviewed by: manu, bz
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42821

(cherry picked from commit 808ae4e29b6b9c9acc7eab013c5045370df8182a)