powerpc/44x: Fix build error with -Werror for Warp platform

With -Werror enabled during the build, the warp.c file fails to build
due to the temp_isr function not containing a return statement.  This
fixes the build error and documents that the function never returns.
Signed-off-by: NJosh Boyer <jwboyer@linux.vnet.ibm.com>

arch/powerpc/platforms/44x/warp.c

@@ -163,6 +163,9 @@ static irqreturn_t temp_isr(int irq, void *context)
 		value ^= 1;
 		mdelay(500);
 	}
+
+	/* Not reached */
+	return IRQ_HANDLED;
 }
 
 static int pika_setup_leds(void)