mac80211: don't check netdev state for debugfs read/write

Doing so will lead to an oops for a p2p-dev interface, since it has
no netdev.

Cc: stable@vger.kernel.org
Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>