[util] Fix argument checks for sparse-fsm-encode.py
[util] Fix argument checks for sparse-fsm-encode.py

This commit changes two argument checks for the sparse-fsm-encode.py
script:
1. Abort if the number of requested states is below 2. The script doesn't
   support this case and previously aborted because of a division or
   modulo by 0.
2. If the number of requested states is 2, the encodings with x bits and
   a minimum Hamming distance of x exists and the script is able to find
   them. Previously, the script would abort and print an error message.

Signed-off-by: Pirmin Vogel <386a7fc8e7338ed7899b4d9883c0ebec5de81366@lowrisc.org>

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