[otbn,dv] Don't send a valid CMD write by accident
[otbn,dv] Don't send a valid CMD write by accident

Be a bit more explicit about what exactly we're randomising when we
choose a value to write to the CMD register. In particular, only the
bottom 8 bits will have any effect.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

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