[ARM] fix drivers/mfd/ucb1x00-core.c IRQ probing bug

While reviewing the IRQ autoprobing code i found the attached buglet.
probe_irq_on()/off() calls must always be in pairs, because the generic IRQ
code uses a global semaphore to serialize all autoprobing activites.
(which does make sense) The ARM code's probe_irq_*() implementation does
not do this, but if this driver is ever used on another platform, this bug
might bite.

(It probably does not trigger in practice, because a zero probing mask
returned should be rare - but still.)
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

drivers/mfd/ucb1x00-core.c

@@ -420,8 +420,10 @@ static int ucb1x00_detect_irq(struct ucb1x00 *ucb)
 	unsigned long mask;
 
 	mask = probe_irq_on();
-	if (!mask)
+	if (!mask) {
+		probe_irq_off(mask);
 		return NO_IRQ;
+	}
 
 	/*
 	 * Enable the ADC interrupt.