powerpc/pmi: Irq handlers return irqreturn_t

Commit bedd30d9 ("genirq: make irqreturn_t
an enum") from the genirq tree in next-20090319 caused this new warning:

arch/powerpc/sysdev/pmi.c: In function 'pmi_of_probe':
arch/powerpc/sysdev/pmi.c:166: warning: passing argument 2 of 'request_irq' from incompatible pointer type

Change the return type of the handler from "int" to "irqreturn_t".

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>