[ARM] CATS: Do not try and map bad PCI IRQ numbers

The cats_map_irq() function in the Simtec CATS support
code is mapping IRQ 255 (invalid IRQ) into what is a
supposedly valid interrupt numner which can cause problems
with other devices then seeing an interrupt they cannot
claim.

If the IRQ number if >= 255, then return -1 as this is not
something we can map.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>