Use per-device debugfs path, so multiple devices won't collide.

in order to use wl->hw->wiphy->debugfsdir, we have to move the debugfs
creation from wl1271_debugfs_init() to wl1271_register_hw().
Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>