Disabling a PCI device should only disable busmaster in the LinuxKPI.

As Linux comment for this function point:
Signal to the system that the PCI device is not in use by the system
anymore. This only involves disabling PCI bus-mastering, if active.

Build tested drm-current-kmod prior to commit.

MFC after: 1 week
Submitted by: slavash@
Sponsored by: Mellanox Technologies