mixer_module: fix poll error in SITL lockstep
mixer_module: fix poll error in SITL lockstep

This fixes the case where the mixer_module would subscribe and use its
own test_motor publication which was created only to make sure the
topic is advertised and subsequent updates will work properly.

This happened in SITL lockstep because the timestamp would be 0 at the
very beginning, and hence elapsed time would be 0 as well.
This lead to an actuator publication which would then get lockstep out
of sync causing poll errors on the Gazebo side.

acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware