wl12xx: remove redundant debugfs_remove_recursive() call

Upon rmmod, the <debugfs>/ieee80211/phyX dir is being removed.
later, we try to remove <debugfs>/ieee80211/phyX/wl12xx, which
might result in NULL dereference.

Remove the excessive debugfs_remove_recursive() call.
(consequently, there is no more need to save wl->rootdir)

Reported-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>