omap: Ptr "isr_reg" tracked as NULL was dereferenced

Value of "isr_reg" pointer is depend on configuration and GPIO method.
Potentially it may have NULL value and it is dereferenced later
in code. Warning and exit from function are added in this case.
Signed-off-by: NEvgeny Kuznetsov <EXT-Eugeny.Kuznetsov@nokia.com>
Signed-off-by: NTony Lindgren <tony@atomide.com>

arch/arm/plat-omap/gpio.c

@@ -1318,6 +1318,10 @@ static void gpio_irq_handler(unsigned int irq, struct irq_desc *desc)
 	if (bank->method == METHOD_GPIO_44XX)
 		isr_reg = bank->base + OMAP4_GPIO_IRQSTATUS0;
 #endif
+
+	if (WARN_ON(!isr_reg))
+		goto exit;
+
 	while(1) {
 		u32 isr_saved, level_mask = 0;
 		u32 enabled;
@@ -1377,6 +1381,7 @@ static void gpio_irq_handler(unsigned int irq, struct irq_desc *desc)
 	configured, we must unmask the bank interrupt only after
 	handler(s) are executed in order to avoid spurious bank
 	interrupt */
+exit:
 	if (!unmasked)
 		desc->chip->unmask(irq);