The checksum option is only relevant if it's not done in "hardware" already.
So it doesn't do anything for Ethernet or USB as these are checksummed already.
On the other hand there was a bug in the JS code greying out this option when
it's not relevant. For some options selecting the radio button was enough
while for some others it only worked if the text on the right was selected.
This is corrected now. Thanks!
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-soci-