[otbn] Avoid an explicit return in an ISS instruction definition
[otbn] Avoid an explicit return in an ISS instruction definition

This looks a bit odd in the generated documentation, so do an if/else
branch instead.

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

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan