tbf: Add and use tbf->poll_ts

Currently tbf->control_ts is used to look up an incoming poll
response. Since that may eventually change, poll timeouts could
theoretically happen in that case.

Store the real poll TS in tbf->poll_ts at all places where
tbf->poll_fn is set. Do not use tbf->control_ts to look up
outstanding polls.

Sponsored-by: On-Waves ehf
diff --git a/src/bts.cpp b/src/bts.cpp
index 61dfc87..0356718 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -307,7 +307,7 @@
 		if (tbf->state_is_not(GPRS_RLCMAC_RELEASING)
 		 && tbf->poll_state == GPRS_RLCMAC_POLL_SCHED
 		 && tbf->poll_fn == fn && tbf->trx->trx_no == trx
-		 && tbf->control_ts == ts) {
+		 && tbf->poll_ts == ts) {
 			return tbf;
 		}
 	}
@@ -325,7 +325,7 @@
 		if (tbf->state_is_not(GPRS_RLCMAC_RELEASING)
 		 && tbf->poll_state == GPRS_RLCMAC_POLL_SCHED
 		 && tbf->poll_fn == fn && tbf->trx->trx_no == trx
-		 && tbf->control_ts == ts) {
+		 && tbf->poll_ts == ts) {
 			return tbf;
 		}
 	}