Add MS time. offset to gsm_lchan

Add MS TIMING OFFSET (3GPP TS 48.058 § 8.4.8) and P offset (3GPP TS
45.010 § 1.2) which can be used to compute MS TO from known TA.

This will be used by osmo-bts (see
I4dfe5c48834a083e757d5de3236a02e15a238b28) to provide MS TO as part of
RSL MEASUREMENT RESULT.

Change-Id: I8bda57c8d6c15bbb803eca708931556dae118a00
Related: OS#1574

openbsc/include/openbsc/gsm_data_shared.h

@@ -327,6 +327,12 @@ struct gsm_lchan {
 		uint8_t last_cmr;
 		uint32_t last_fn;
 	} tch;
+
+	/* 3GPP TS 48.058 § 9.3.37: [0; 255] ok, -1 means invalid*/
+	int16_t ms_t_offs;
+	/* 3GPP TS 45.010 § 1.2 round trip propagation delay (in symbols) or -1 */
+	int16_t p_offs;
+
 	/* BTS-side ciphering state (rx only, bi-directional, ...) */
 	uint8_t ciph_state;
 	uint8_t ciph_ns;