specialix.c: fix possible double-unlock

Noticed by sparse, trivial to see:
drivers/char/specialix.c:2112:3: warning: context imbalance in 'sx_throttle' - unexpected unlock

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>