iwlwifi: clean up debugfs code

The debugfs code can be made a whole lot more
efficient by using debugfs_remove_recursive(),
the large chunk of variables can completely go
away and by moving two variables we no longer
need to allocate an extra chunk of memory.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>