On machines using the "ISU" mecanism for the MPIC, the new driver didn't properly
calculate the new interrupt count when an ISU was added. That would cause later on
failure to request interrupts in the offending range.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>