ath9k: Document keycache operations

There are number of small details about the keycache operations that
are very easy to miss (and forget), so better include detailed
comments in ath9k_hw_set_keycache_entry() to avoid having to figure
out this every time when having to touch this area.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath9k/mac.h

@@ -588,9 +588,11 @@ struct ath9k_keyval {
 	u8 kv_type;
 	u8 kv_pad;
 	u16 kv_len;
-	u8 kv_val[16];
-	u8 kv_mic[8];
-	u8 kv_txmic[8];
+	u8 kv_val[16]; /* TK */
+	u8 kv_mic[8]; /* Michael MIC key */
+	u8 kv_txmic[8]; /* Michael MIC TX key (used only if the hardware
+			 * supports both MIC keys in the same key cache entry;
+			 * in that case, kv_mic is the RX key) */
 };
 
 enum ath9k_key_type {