fixed incorrect formatting of enum values on soft signal set

[evalott100]

Currently the code incorrectly formats enum put values into their string version on SoftSignalBackend leading to the following bug:

    soft_signal = SoftSignalBackend(MyEnum)
    await soft_signal.connect()
    assert (await soft_signal.get_value()) is MyEnum.a
    await soft_signal.put(MyEnum.b)
    # fails, the value is now the string form
    assert (await soft_signal.get_value()) is MyEnum.b
    

@DiamondJoseph This was the cause of the error in the tetramm tests DiamondLightSource/dodal#649