tbf: Update the legend and mention invalid as well
We should not see it but the legend should be complete.
diff --git a/src/tbf.cpp b/src/tbf.cpp
index c975cee..a75fe4a 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -1406,7 +1406,7 @@
mod_sns, mod_sns_half);
LOGP(DRLCMACDL, LOGL_DEBUG, "- V(B): (V(A)=%d)\"%s\""
"(V(S)-1=%d) A=Acked N=Nacked U=Unacked "
- "X=Resend-Unacked\n", dir.dl.v_a, show_v_b,
+ "X=Resend-Unacked I=Invalid\n", dir.dl.v_a, show_v_b,
(dir.dl.v_s - 1) & mod_sns);
if (state_is(GPRS_RLCMAC_FINISHED) && dir.dl.v_s == dir.dl.v_a) {