softdevice: don't send a notify/indicate without a CCCD

sd_ble_gatts_hvx_noescape can only be called when the notify/indicate
permission is set (and therefore a CCCD has been added). Without it, it
will just return an error.

This fixes a problem I found on the PineTime, while implementing the
battery service.

gatts_sd.go

@@ -118,7 +118,7 @@ func (c *Characteristic) Write(p []byte) (n int, err error) {
 	}
 
 	connHandle := currentConnection.Get()
-	if connHandle != C.BLE_CONN_HANDLE_INVALID {
+	if connHandle != C.BLE_CONN_HANDLE_INVALID && c.permissions&(CharacteristicNotifyPermission|CharacteristicIndicatePermission) != 0 {
 		// There is a connected central.
 		p_len := uint16(len(p))
 		errCode := C.sd_ble_gatts_hvx_noescape(connHandle,