iwlwifi: mvm: check firmware is up in debugfs

Protect various debugfs files that need to communicate with the
firmware from being used when the firmware isn't running.

Some will just reject getting written to, while others that store
some state will simply store it and not apply it immediately.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>