powerpc/pseries: Make 32-bit MSI quirk work on systems lacking firmware support

powerpc/pseries: Make 32-bit MSI quirk work on systems lacking firmware support

Recent commit a83b89631f10aae0847dc38d8590b4021b5e4b13 added support
for a new firmware feature to force an adapter to use 32 bit MSIs.
However, this firmware is not available for all systems. The hack below
allows devices needing 32 bit MSIs to work on these systems as well.
It is careful to only enable this on Gen2 slots, which should limit
this to configurations where this hack is needed and tested to work.

[Small change to factor out the hack into a separate function -- BenH]

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>