tbf: Mark initial first_(common_)ts with special value

This way it's easier to distinguish when this value was not properly
filled when debugging or looking at logs.

Change-Id: I0c9c9fdcfca9eb15125ea49efcbb76711850052e
diff --git a/src/tbf.h b/src/tbf.h
index 4013ab0..4652d95 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -116,7 +116,7 @@
 #define GPRS_RLCMAC_FLAG_TO_DL_ACK	3 /* DL TBF: Failed to receive last polled DL ACK/NACK */
 #define GPRS_RLCMAC_FLAG_TO_MASK	0xf0 /* timeout bits */
 
-#define TBF_CONTROL_TS_UNSET 0xff
+#define TBF_TS_UNSET 0xff
 
 #define T_START(tbf, t, T, r, f) tbf->t_start(t, T, r, f, __FILE__, __LINE__)