Changes

e1_ts_stop: clear tx_started flag

ts->raw.tx_started flag gets set in the channelized mux process
when bytes begin arriving from the raw TS client application.
However, there is no reset of this flag anywhere, hence if a user
opens some TS, then kills that client application, then opens it
again (with the same or different client app), there is a flood
of error messages about TS read underflow.  Solution: clear the
tx_started flag when the TS is stopped.

Change-Id: I28ff68208f1d3d20dc57c5306fec74bd2f884cb1