Fix the OFW interrupt map parser to use its own idea of the number of interrupt

Fix the OFW interrupt map parser to use its own idea of the number of interrupt
cells in the map, instead of using a value passed to it and then panicing if it
disagrees. This fixes interrupt map parsing for PCI bridges on some Apple
Uninorth PCI controllers.

Reported by:	marcel
Tested on:	G4 iBook, Sun Ultra 5