MIPS: IP32: Fix hang on shutdown in power button interrupt handler.

The hang was caused by the use of disable_irq() from the interrupt handler
itself.  Fixed by the use of disable_irq_nosync().  The issue was
triggered by:

    commit 3aa551c9
    Author: Thomas Gleixner <tglx@linutronix.de>
    Date:   Mon Mar 23 18:28:15 2009 +0100

        genirq: add threaded interrupt handler support
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/sgi-ip32/ip32-reset.c

@@ -145,7 +145,7 @@ static irqreturn_t ip32_rtc_int(int irq, void *dev_id)
 			"%s: RTC IRQ without RTC_IRQF\n", __func__);
 	}
 	/* Wait until interrupt goes away */
-	disable_irq(MACEISA_RTC_IRQ);
+	disable_irq_nosync(MACEISA_RTC_IRQ);
 	init_timer(&debounce_timer);
 	debounce_timer.function = debounce;
 	debounce_timer.expires = jiffies + 50;