[MIPS] SMTC: Make ack_bad_irq() safe with no IM backstop.

Issue reported and original patch by Kevin Kissel, cleaner (imho)
implementation by me.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/kernel/irq.c b/arch/mips/kernel/irq.c
index aeded6c..a990aad 100644
--- a/arch/mips/kernel/irq.c
+++ b/arch/mips/kernel/irq.c
@@ -74,20 +74,12 @@
  */
 void ack_bad_irq(unsigned int irq)
 {
+	smtc_im_ack_irq(irq);
 	printk("unexpected IRQ # %d\n", irq);
 }
 
 atomic_t irq_err_count;
 
-#ifdef CONFIG_MIPS_MT_SMTC
-/*
- * SMTC Kernel needs to manipulate low-level CPU interrupt mask
- * in do_IRQ. These are passed in setup_irq_smtc() and stored
- * in this table.
- */
-unsigned long irq_hwmask[NR_IRQS];
-#endif /* CONFIG_MIPS_MT_SMTC */
-
 /*
  * Generic, controller-independent functions:
  */