drivers/macintosh/smu.c: fix locking snafu

It got its lock and unlock backwards.

Fixes http://bugzilla.kernel.org/show_bug.cgi?id=8334



(obviously, this code could be using plain old spin_lock_irq(), too)

Cc: <matthias.kaehlcke@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>