mfd: pcf50633: fix unsafe disable_irq()

Without this change Openmoko Freerunner (GTA02) bootstrap will deadlock.
As pointed out in other patches this issue is in the wild since the merge
of:

: commit 3aa551c9


: Author: Thomas Gleixner <tglx@linutronix.de>
: Date:   Mon Mar 23 18:28:15 2009 +0100
:
:    genirq: add threaded interrupt handler support
:
:    Add support for threaded interrupt handlers

Signed-off-by: Nelson Castillo <arhuaco@freaks-unidos.net>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: <balajirrao@openmoko.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>