dtx_check.gawk: Fix false-positives in DTX check
dtx_check.gawk: Fix false-positives in DTX check

Previously, if ONSET happened exactly at the place where next SID FIRST
or UPDATE should be it was incorrectly detected as error. Similarly,
continuos FACCH was misinterpreted as error.

Change-Id: I43fdbceea0dbdb0833c3b1cf0fc3b825803ed30e

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts