This will be used by more than MVM, so move it to iwlwifi

While at it, rename WRD_METHOD to the more appropriate
WRDD_METHOD and add some documentation.
Signed-off-by: NArik Nemtsov <arikx.nemtsov@intel.com>
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>
Signed-off-by: NLuca Coelho <luciano.coelho@intel.com>