ntb_hw_intel: fix xeon NTB gen3 bar disable logic

In NTB gen3 driver, it was supposed to disable NTB bar access by
default, but due to incorrect register access method, the bar disable
logic does not work as expected. Those registers should be modified
through NTB bar0 rather than PCI configuration space.

Besides, we'd better to protect ourselves from a bad buddy node so
ingress disable logic should be implemented together.

Submitted by: Austin Zhang (austin.zhang@dell.com)
Reviewers: markj, mav, vangyzen, dab
Differential Revision: https://reviews.freebsd.org/D31736
Sponsored by: Dell EMC
MFC to: stable/12, stable/13
MFC after: 1 week