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: Evgeny Kuznetsov <EXT-Eugeny.Kuznetsov@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
diff --git a/arch/arm/plat-omap/gpio.c b/arch/arm/plat-omap/gpio.c
index c05c653..e0e2fa7 100644
--- a/arch/arm/plat-omap/gpio.c
+++ b/arch/arm/plat-omap/gpio.c
@@ -1318,6 +1318,10 @@
 	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 @@
 	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);