DTX: add explicit check if DTX enabled
DTX: add explicit check if DTX enabled

Check explicitly if DTX is enabled for AMR before checking if SID frame
repetition is optional.

Change-Id: I660688d56505798cade1495c30338fd6806a3259
Related: OS#1801

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