Setup rfxtrx event listener directly

[elupus]

Should not be problem now that we use dispatcher for other events.

Proposed change

No point in waiting for start event anymore since we use dispatcher

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

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to pull: Create option flow for Rfxtrx integration #37982
• 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
• 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.

The integration reached or maintains the following Integration Quality Scale:

• No score or internal
• 🥈 Silver
• 🥇 Gold
• 🏆 Platinum

[elupus]

I can't see a reason to wait for hass.state == running, so i skipped doing it.

[probot-home-assistant]

Hey there @Danielhiversen, mind taking a look at this pull request as its been labeled with an integration (rfxtrx) you are listed as a codeowner for? Thanks!
_{^{(message by CodeOwnersMention)}}