Revert "tbf: Use the control TS for Immediate Assignments"

According to spec, the lowest numbered PDCH of the TBF shall be used
for the PACCH, only when concurrent TBF are active, the lowest common
PDCH shall be used. An immediate assigment is in general only sent,
if no TBF is active, so the latter case never applies.

This reverts commit fdd8a1ba2312be3f33576ee994c0b5da6272e815.
diff --git a/src/bts.cpp b/src/bts.cpp
index 12f4e72..0d3e32a 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -525,7 +525,7 @@
 		LOGP(DRLCMAC, LOGL_INFO, "%s TX: START Immediate "
 			"Assignment Uplink (AGCH)\n", tbf_name(tbf));
 		trx_no = tbf->trx->trx_no;
-		ts_no = tbf->control_ts;
+		ts_no = tbf->first_ts;
 		tfi = tbf->tfi();
 		usf = tbf->m_usf[ts_no];
 		tsc = tbf->tsc();
@@ -585,7 +585,7 @@
 void BTS::snd_dl_ass(gprs_rlcmac_tbf *tbf, uint8_t poll, const char *imsi)
 {
 	int plen;
-	unsigned int ts = tbf->control_ts;
+	unsigned int ts = tbf->first_ts;
 
 	LOGP(DRLCMAC, LOGL_INFO, "TX: START %s Immediate Assignment Downlink (PCH)\n", tbf_name(tbf));
 	bitvec *immediate_assignment = bitvec_alloc(22); /* without plen */