Update mysensors to use async_add_executor_job

[bdraco]

Proposed change

Update mysensors to use async_add_executor_job

There was a check to see if a device is a serial port that
did not, AFAICT, do IO and did not need to be run in the executor
It's actually calling os.stat(value) under the hood.

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[probot-home-assistant]

Hey there @MartinHjelmare, mind taking a look at this pull request as its been labeled with an integration (mysensors) you are listed as a codeowner for? Thanks!
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