powerpc: Fix i8259 interrupt driver kernel crash on ML510

This patch fixes a null pointer exception caused by removal of
'ack()' for level interrupts in the Xilinx interrupt driver.  A recent
change to the xilinx interrupt controller removed the ack hook for
level irqs.

Signed-off-by: Roderick Colenbrander <thunderbird2k@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/powerpc/sysdev/xilinx_intc.c

@@ -234,7 +234,6 @@ static void xilinx_i8259_cascade(unsigned int irq, struct irq_desc *desc)
 		generic_handle_irq(cascade_irq);
 
 	/* Let xilinx_intc end the interrupt */
-	desc->chip->ack(irq);
 	desc->chip->unmask(irq);
 }