[ARM] Fix Neponset IRQ handling

While testing the genirq code on ARM, a condition was found whereby
the Neponset IRQ handler was being re-entered, causing the system
to deadlock.

Under the ARM IRQ code, this would not have been a visible problem
because the "simple" IRQ handling had no re-entrancy protection.

Resolve this by acknowledging the parent interrupt after we mask it
when we are going to handle one of our "special" level-based sources
(from ethernet or USAR chip.)
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mach-sa1100/neponset.c

@@ -59,6 +59,14 @@ neponset_irq_handler(unsigned int irq, struct irqdesc *desc, struct pt_regs *reg
 		if (irr & (IRR_ETHERNET | IRR_USAR)) {
 			desc->chip->mask(irq);
 
+			/*
+			 * Ack the interrupt now to prevent re-entering
+			 * this neponset handler.  Again, this is safe
+			 * since we'll check the IRR register prior to
+			 * leaving.
+			 */
+			desc->chip->ack(irq);
+
 			if (irr & IRR_ETHERNET) {
 				d = irq_desc + IRQ_NEPONSET_SMC9196;
 				desc_handle_irq(IRQ_NEPONSET_SMC9196, d, regs);