[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>