m32r: Fixup last __do_IRQ leftover

Somehow I managed to miss the last __do_IRQ caller when I cleanup the
remaining users. m32r is fully converted to the generic irq layer, but
I managed to not commit the conversion of __do_IRQ() to
generic_handle_irq() after compile testing the quilt series :(

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