[otbn] Don't send RND requests on an IMEM error
[otbn] Don't send RND requests on an IMEM error

If the check bits for an instruction are invalid, we shouldn't take
any outwardly observable action. In this case, we weren't squashing
the RND request from a CSRRW that wrote to RND_PREFETCH.

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

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