trx_interface: implement built-in timeslot filter

There is a dedicated block named 'Burst Timeslot Filter', which
could be used, but one doesn't expose the API to change the
timeslot index 'on fly'.

This change implements built-in timeslot filter in 'TRX Interface'
block, which will be used until the proper block expose API.
diff --git a/lib/trx_interface/trx_impl.h b/lib/trx_interface/trx_impl.h
index 65b4d97..19e4848 100644
--- a/lib/trx_interface/trx_impl.h
+++ b/lib/trx_interface/trx_impl.h
@@ -36,6 +36,7 @@
      private:
       udp_socket *d_data_sock;
       udp_socket *d_clck_sock;
+      int d_ts_filter_tn;
 
       bool detect_rach(uint8_t *burst);
       void clck_ind_send(uint32_t frame_nr);
@@ -45,6 +46,10 @@
       trx_impl(const std::string &remote_addr, int base_port);
       ~trx_impl();
 
+      /* Timeslot filter API */
+      void ts_filter_set_tn(int tn);
+      int ts_filter_get_tn(void);
+
       void handle_dl_burst(pmt::pmt_t msg);
       void handle_ul_burst(uint8_t *payload, size_t len);
     };