Set volume_step in cmus media_player

[emontnemery]

Proposed change

Set volume_step in cmus media_player

Repeated calls to volume up/down service will work: cmus media player is polled, which means update is called after each service call

Background in PR #105574

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[emontnemery]

I'm not sure how crucial this calculation of an average volume is for the volume up / down.

[MartinHjelmare]

The averaging is already done in the state update for _attr_volume_level so the base implementation of volume up/down will have the same behavior.