acer-wmi: remove debugfs entries upon unloading

The exit function neglects to remove debugfs entries, leading to a BUG
on reload.

[akpm@linux-foundation.org: cleanups]
Signed-off-by: Russ Dill <Russ.Dill@gmail.com>
Acked-by: Carlos Corbacho <carlos@strangeworlds.co.uk>
Cc: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>