[POWERPC] Fix warning in prom_parse.c of_irq_map_oldworld()

This function spews a warning due to possible use of an uninitialized
variable. This can happen on broken device-trees or when called with
a NULL argument. Makes ure we properly fail instead.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>