sched: irq: Refactor irq_spinlock.c

Summary:
- Call this_cpu() properly in spin_unlock_irqrestore()

Impact:
- None

Testing:
- Tested with spresense:wifi_smp

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

sched/irq/irq_spinlock.c

@@ -137,10 +137,9 @@ irqstate_t spin_lock_irqsave(spinlock_t *lock)
 
 void spin_unlock_irqrestore(spinlock_t *lock, irqstate_t flags)
 {
-  int me = this_cpu();
-
   if (NULL == lock)
     {
+      int me = this_cpu();
       DEBUGASSERT(0 < g_irq_spin_count[me]);
       g_irq_spin_count[me]--;