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 3aa551c9b4c40018f0e261a178e3d25478dc04a9
: Author: Thomas Gleixner <[e-mail]>
: 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 <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Samuel Ortiz <[e-mail]>

drivers/mfd/pcf50633-core.c

···
 	dev_dbg(pcf->dev, "pcf50633_irq\n");
 
 	get_device(pcf->dev);
-	disable_irq(pcf->irq);
+	disable_irq_nosync(pcf->irq);
 	schedule_work(&pcf->irq_work);
 
 	return IRQ_HANDLED;