blackfin idle: Fix compile error

Commit 26bab0c9 ("blackfin idle: delete pm_idle") introduced the following
compile error:
	arch/blackfin/kernel/process.c: In function ‘cpu_idle’:
	arch/blackfin/kernel/process.c:83: error: ‘idle’ undeclared (first use in this function)
	arch/blackfin/kernel/process.c:83: error: (Each undeclared identifier is reported only once
	arch/blackfin/kernel/process.c:83: error: for each function it appears in.)
	arch/blackfin/kernel/process.c:88: error: implicit declaration of function ‘idle’

This patch fixes it.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Len Brown <len.brown@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

arch/blackfin/kernel/process.c

@@ -80,12 +80,10 @@ void cpu_idle(void)
 		if (cpu_is_offline(smp_processor_id()))
 			cpu_die();
 #endif
-		if (!idle)
-			idle = default_idle;
 		tick_nohz_idle_enter();
 		rcu_idle_enter();
 		while (!need_resched())
-			idle();
+			default_idle();
 		rcu_idle_exit();
 		tick_nohz_idle_exit();
 		preempt_enable_no_resched();