m32r: Cleanup direct irq_desc access

The irq descriptors are already initialized by the generic
code. Remove the redundant init code and set the irq chip with the
proper accessor function.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Hirokazu Takata <takata@linux-m32r.org>
Cc: Paul Mundt <lethal@linux-sh.org>