nfc: st21nfcb: Add additional comments about EVT_TRANSACTION

Add comments about HCI EVT_TRANSACTION in order to make the code
understandable by other readers.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/nfc/st21nfcb/st21nfcb_se.c

@@ -321,6 +321,12 @@ static int st21nfcb_hci_connectivity_event_received(struct nci_dev *ndev,
 
 	break;
 	case ST21NFCB_EVT_TRANSACTION:
+		/* According to specification etsi 102 622
+		 * 11.2.2.4 EVT_TRANSACTION Table 52
+		 * Description  Tag     Length
+		 * AID          81      5 to 16
+		 * PARAMETERS   82      0 to 255
+		 */
 		if (skb->len < NFC_MIN_AID_LENGTH + 2 &&
 		    skb->data[0] != NFC_EVT_TRANSACTION_AID_TAG)
 			return -EPROTO;
@@ -329,8 +335,9 @@ static int st21nfcb_hci_connectivity_event_received(struct nci_dev *ndev,
 					    skb->len - 2, GFP_KERNEL);
 
 		transaction->aid_len = skb->data[1];
-		memcpy(transaction->aid, &skb->data[2], skb->data[1]);
+		memcpy(transaction->aid, &skb->data[2], transaction->aid_len);
 
+		/* Check next byte is PARAMETERS tag (82) */
 		if (skb->data[transaction->aid_len + 2] !=
 		    NFC_EVT_TRANSACTION_PARAMS_TAG)
 			return -EPROTO;