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>