lms: Drop rx_underruns rate ctr, add tx_drop_* rate ctr

After discussion in [1] and further look at the code, it became obvios
rx_underrun events are not happening in general for any SDR (don't
exist), so let's drop that counter. Instead, add Tx Dropped Packet counters,
which were not accounted prior to this commit.

[1] https://github.com/osmocom/osmo-trx/commit/bde55afd29fc9aae10eb11f6515821afa39b772d

Change-Id: Iff1535c219a4695a511d383d7c4b06ef6eff959d
diff --git a/CommonLibs/osmo_signal.h b/CommonLibs/osmo_signal.h
index ee7e2a4..ceb7d6f 100644
--- a/CommonLibs/osmo_signal.h
+++ b/CommonLibs/osmo_signal.h
@@ -48,9 +48,10 @@
 /* signal cb for signal <SS_DEVICE,S_DEVICE_COUNTER_CHANGE> */
 struct device_counters {
 	size_t chan;
-	unsigned int rx_underruns;
 	unsigned int rx_overruns;
 	unsigned int tx_underruns;
 	unsigned int rx_dropped_events;
 	unsigned int rx_dropped_samples;
+	unsigned int tx_dropped_events;
+	unsigned int tx_dropped_samples;
 };