iwlagn: move sysfs flags and filter_flags files to debugfs

These files are incompatible with some changes
I'm making, and don't really belong into sysfs
anyway as they can only be used for debugging.
Since writing them will probably crash the
firmware, keep only the reading part and move
them into debugfs.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>