tbf: Always increase the tx_counter when we transmit a frame

In case the ACK could not be scheduled we were not increasing the
tx_counter. Change the code flow to always increase the tx_counter
after we have created a frame.
diff --git a/src/tbf.cpp b/src/tbf.cpp
index baa01ef..b76a5ec 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -1181,19 +1181,17 @@
 			/* set polling in header */
 			rh->rrbp = 0; /* N+13 */
 			rh->s_p = 1; /* Polling */
-
-			/* Increment TX-counter */
-			dir.dl.tx_counter++;
 		}
-	} else {
-		/* Increment TX-counter */
-		dir.dl.tx_counter++;
 	}
 
 	/* return data block as message */
 	dl_msg = msgb_alloc(len, "rlcmac_dl_data");
 	if (!dl_msg)
 		return NULL;
+
+	/* Increment TX-counter */
+	dir.dl.tx_counter++;
+
 	memcpy(msgb_put(dl_msg, len), data, len);
 	bts->rlc_sent();