pci: don't disable too many HT MSI mapping

Prakash's system needs MSI disabled on some bridges, but not all.
This seems to be the minimal fix for 2.6.29, but should be replaced
during 2.6.30.

Signed-off-by: Prakash Punnoor <prakash@punnoor.de>
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>