fw: make sure to clear the timer IRQ counter on init...

.sbss isn't being cleared on soft reset anymore...

fw/hello.c

@@ -145,6 +145,7 @@ static void periodic_irq(unsigned int vector)
     set_led(count >> 3); /* 4 Hz */
 }
 
+static uint32_t timer_irq_start;
 static void init(void)
 {
     static const char hello[] =
@@ -156,6 +157,8 @@ static void init(void)
 
     con_puts(hello);
 
+    timer_irq_count = 0;
+    timer_irq_start = rdtime();
     register_irq(3, periodic_irq, true);
 }
 
@@ -184,7 +187,7 @@ void main(void)
     scrub_sdram();
 
     irq_count = timer_irq_count;
-    done = rdtime() - time_zero;
+    done = rdtime() - timer_irq_start;
 
     con_printf("%u timer interrupts received in %u us\n",
 	       irq_count, done/(CPU_HZ/1000000));