Merge pull request #533 from TinyHai/fix · stm32-rs/stm32f1xx-hal@aafd9c2

creating a shared reference to mutable static is discouraged

`stm32f1xx-hal` (example "serial-interrupt-idle") generated 4 warnings

creating a mutable reference to mutable static is discouraged

creating a mutable reference to mutable static is discouraged

creating a mutable reference to mutable static is discouraged

creating a mutable reference to mutable static is discouraged

creating a mutable reference to mutable static is discouraged

creating a mutable reference to mutable static is discouraged

creating a mutable reference to mutable static is discouraged

creating a mutable reference to mutable static is discouraged

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