NFC: st21nfca: Implement stop_poll HCI hook

Send DM_DISCONNECT command to disconnect Terminal Host from the HCI network.

- The persistent states of the terminal host pipes, including registry values,
are not modifies. Therefore, there is no NVRAM update to disconnect the
terminal host.
- The terminal host RF card gates are disabled which means that there will be no event
related to card RF gates until communication has been restored.
- The terminal host RF reader request is reset so the RF reader polling for terminal
host is disabled.

To restore the communication, the terminal host can send any HCI command or event.

Signed-off-by: Christophe Ricard <[e-mail]>
Signed-off-by: Samuel Ortiz <[e-mail]>

drivers/nfc/st21nfca/st21nfca.c

···
 #define ST21NFCA_DM_PIPE_CREATED        0x02
 #define ST21NFCA_DM_PIPE_OPEN           0x04
 #define ST21NFCA_DM_RF_ACTIVE           0x80
+#define ST21NFCA_DM_DISCONNECT		0x30
 
 #define ST21NFCA_DM_IS_PIPE_OPEN(p) \
 	((p & 0x0f) == (ST21NFCA_DM_PIPE_CREATED | ST21NFCA_DM_PIPE_OPEN))
···
 	return r;
 }
 
+static void st21nfca_hci_stop_poll(struct nfc_hci_dev *hdev)
+{
+	nfc_hci_send_cmd(hdev, ST21NFCA_DEVICE_MGNT_GATE,
+			ST21NFCA_DM_DISCONNECT, NULL, 0, NULL);
+}
+
 static int st21nfca_get_iso14443_3_atqa(struct nfc_hci_dev *hdev, u16 *atqa)
 {
 	int r;
···
 	.hci_ready = st21nfca_hci_ready,
 	.xmit = st21nfca_hci_xmit,
 	.start_poll = st21nfca_hci_start_poll,
+	.stop_poll = st21nfca_hci_stop_poll,
 	.target_from_gate = st21nfca_hci_target_from_gate,
 	.im_transceive = st21nfca_hci_im_transceive,
 	.check_presence = st21nfca_hci_check_presence,