m32r: Use preempt_schedule_irq

Use the proper core function instead of fiddling with preempt_active
and interrupt enable in the low level code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Hirokazu Takata <takata@linux-m32r.org>
Cc: linux-m32r-ja@ml.linux-m32r.org
Link: http://lkml.kernel.org/r/20130917183628.758421136@linutronix.de