Fix RampDownAdapter handling repeating waveforms

The adapter replaces the zero-amplitude steps with a ramp down to zero
on devices with configured ramp down duration, but does not handle well
the repeat index and ends up repeating to a part of a ramp introduced.

This fixes the repeat index handling and also the edge case when the
replaced zero segment is the one it starts repeating.

Fix: 198755610
Test: RampDownAdapterTest
Change-Id: I2489b1a73088f2b992455f2b687daf5c54298dd1