Adjust default value for volume down step

[mathebox]

This pull request adjusts the default value for the volume step down to a positive integer.
Config flow requires the value in a range of positive integers.

teufel_raumfeld/custom_components/teufel_raumfeld/config_flow.py

Lines 96 to 102 in 786cd54

	vol.Required(
	OPTION_CHANGE_STEP_VOLUME_DOWN,
	default=self.config_entry.options.get(
	OPTION_CHANGE_STEP_VOLUME_DOWN,
	DEFAULT_CHANGE_STEP_VOLUME_DOWN,
	),
	): vol.All(int, vol.Range(min=1, max=20)),

Afterward, this value is multiplied by -1 for the correct volume change.

teufel_raumfeld/custom_components/teufel_raumfeld/media_player.py

Line 649 in 786cd54

-1 * self._raumfeld.options[OPTION_CHANGE_STEP_VOLUME_DOWN]

Hence, the default value needs to be a positive integer as well.

Fixes #54