[Y-Cable][Credo] add theading locker to support thread-safe calling, …

…add SKU check for download_firmware API. (sonic-net#222)

* [Y-Cable][Credo] fix racing issue of VSC releated APIs

    The VSC protocol didn't allow user to send two or more vsc command to the module simultaneously,
    otherwise unexpected error might occurred.

    To avoid this issue, we can simply to update the download_firmware_status in the the following functions.
    This could help the helper to know vsc is in progress or not.

        * get_firmware_version()
        * download_firmware()
        * activate_firmware()
        * rollback_firmware()
This PR also does add SKU check for download_firmware API.
* [Y-Cable][Credo] add firmware ID checker to avoid update the wrong firmware

Signed-off-by: xinyu <xinyu0123@gmail.com>