Fix Timing Advance handling

* initialize with invalid TA instead of making assumption that phone is
  located within 550 meters (TA=0)
* only set valid TA

Change-Id: Idfc40ff0c11bdac13d9e28fbfa4e95dfc6b735b0
Related: OS#1526
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index 4c67a12..489020b 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -120,7 +120,7 @@
 {
 	uint8_t ss;
 	int8_t use_trx;
-	uint16_t ta = 0;
+	uint16_t ta = GSM48_TA_INVALID;
 	struct gprs_rlcmac_ul_tbf *ul_tbf = NULL, *old_ul_tbf;
 	struct gprs_rlcmac_dl_tbf *dl_tbf = NULL;
 	GprsMs *ms;