PPC64: Partial fix to Linux crash: revert to old devfn

Before 2e29bd04786003561303dcad940b38afe790fb9b, Uni-North host and AGP
bridge were using the same devfn, 11 << 3. The net effect was that only
AGP bridge was visible. This was changed in the commit to 13 << 3 for AGP.

Some of the later crashes happened due to NULL pointer dereferences generated
by 07b7d05377a5e2b242ef0cce3d461d3284700fc0 and insufficient return value
checks.

Disable host device and revert AGP to 11 << 3.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
