Use Timing Advance Index in UL assignments

Write TAI (if available) when generating Rest Octets for UL
Assignment. This should not affect actual PCU behavior because TAI is
not yet supported by upper layers but we have to adjust corresponding
tests anyway.

Change-Id: I8b17be78a46c0bc17516b7c90f35aa4768010ae4
diff --git a/src/encoding.cpp b/src/encoding.cpp
index e96894f..39cbe24 100644
--- a/src/encoding.cpp
+++ b/src/encoding.cpp
@@ -255,7 +255,9 @@
 		rc = write_alpha_gamma(dest, alpha, gamma);
 		check(rc);
 
-		set_0(dest); /* No TIMING_ADVANCE_INDEX */
+		rc = write_ta_index(dest, ta_idx);
+		check(rc);
+
 		set_0(dest); /* No TBF_STARTING_TIME */
 	} else {
 		set_0(dest); /* Single Block Allocation */
@@ -317,7 +319,9 @@
 		rc = write_alpha_gamma(dest, alpha, gamma);                 /* ALPHA and GAMMA */
 		check(rc);
 
-		set_0(dest);                                                /* No TIMING_ADVANCE_INDEX */
+		rc = write_ta_index(dest, ta_idx);
+		check(rc);
+
 		set_0(dest);                                                /* No TBF_STARTING_TIME */
 		bitvec_set_bit(dest, L);                                    /* No Additions for Rel-7 */
 	} else {