gpio: mpc8xxx: Prevent NULL pointer deref in demux handler

commit cfadd838(powerpc/8xxx: Fix interrupt handling in MPC8xxx GPIO
driver) added an unconditional call of chip->irq_eoi() to the demux
handler.

This leads to a NULL pointer derefernce on MPC512x platforms which use
this driver as well.

Make it conditional.
Reported-by: Thomas Wucher <thwucher@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Felix Radensky <felix@embedded-sol.com>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: stable@vger.kernel.org
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/gpio/gpio-mpc8xxx.c

@@ -163,7 +163,8 @@ static void mpc8xxx_gpio_irq_cascade(unsigned int irq, struct irq_desc *desc)
 	if (mask)
 		generic_handle_irq(irq_linear_revmap(mpc8xxx_gc->irq,
 						     32 - ffs(mask)));
-	chip->irq_eoi(&desc->irq_data);
+	if (chip->irq_eoi)
+		chip->irq_eoi(&desc->irq_data);
 }
 
 static void mpc8xxx_irq_unmask(struct irq_data *d)