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 <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Cc: Hirokazu Takata <[e-mail]>
Cc: Paul Mundt <[e-mail]>

arch/m32r/kernel/irq.c

···
 #ifdef CONFIG_DEBUG_STACKOVERFLOW
 	/* FIXME M32R */
 #endif
-	__do_IRQ(irq);
+	generic_handle_irq(irq);
 	irq_exit();
 	set_irq_regs(old_regs);