ath5k: remove permissions from debugfs files

Don't allow users to open debugfs files, because it can cause oopses.

When a user opens some file, driver unlinks it and frees the
corresponding structure, we will dereference freed memory.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>