Polynomial division LFSR
I think what you want is to have a multiplexer between the IN bit and the LFSR which will route the input bit around unneeded LFSR stages. This would require lines to not only deactivate unused stages but to reroute the output bit from one stage to another. It's possible but is going to be fairly hardware expensive. What is the purpose of having a variable-degree LFSR in hardware?