hw/openrisc: Avoid undefined shift in openrisc_pic_cpu_handler()

In C99 signed shift (1 << 31) is undefined behavior, since the result
exceeds INT_MAX.  Use 1U instead and move the shift after the check.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Acked-by: Jia Liu <proljc@gmail.com>

hw/openrisc/pic_cpu.c

@@ -26,12 +26,14 @@ static void openrisc_pic_cpu_handler(void *opaque, int irq, int level)
 {
     OpenRISCCPU *cpu = (OpenRISCCPU *)opaque;
     CPUState *cs = CPU(cpu);
-    uint32_t irq_bit = 1 << irq;
+    uint32_t irq_bit;
 
     if (irq > 31 || irq < 0) {
         return;
     }
 
+    irq_bit = 1U << irq;
+
     if (level) {
         cpu->env.picsr |= irq_bit;
     } else {