Fix volume calculation of Onkyo MediaPlayer

[ZzetT]

Proposed change

If the max_volume is configured for the Onkyo integration and the user sets a new volume in the media player, the volume is correctly transmitted to the receiver, but when update() of that player is called, the volume (returned by volume_level()) is calculated wrongly and thus the slider in the media player jumps to another position. In fact, the formula misses some parentheses.
I wrote the details down in: #41655 (comment)

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example entry for configuration.yaml:

# Example configuration.yaml
media_player:
  - platform: onkyo
    host: 192.168.178.30
    name: Onkyo Receiver
    max_volume: 33

Additional information

• This PR fixes or closes issue: fixes Onkyo media_player volume_level issue when max_volume set #42090 and MediaPlayer Onkyo Volume Scale #41655
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
  IMHO the integration doesn't have any tests
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

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[MartinHjelmare]

Thanks!