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
8 files changed