mac80211: Get IV len from key conf and not cipher scheme

When a key is installed using a cipher scheme, set a new
internal key flag (KEY_FLAG_CIPHER_SCHEME) on it, to allow
distinguishing such keys more easily.

In particular, use this flag on the TX path instead of
testing the sta->cipher_scheme pointer, as the station is
NULL for broad-/multicast message, and use the key's iv_len
instead of the cipher scheme information.

Signed-off-by: Cedric Izoard <cedric.izoard@ceva-dsp.com>
[add missing documentation, rewrite commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/key.c

@@ -492,6 +492,7 @@ ieee80211_key_alloc(u32 cipher, int idx, size_t key_len,
 				for (j = 0; j < len; j++)
 					key->u.gen.rx_pn[i][j] =
 							seq[len - j - 1];
+			key->flags |= KEY_FLAG_CIPHER_SCHEME;
 		}
 	}
 	memcpy(key->conf.key, key_data, key_len);