[otbn] Fully mux between model and RTL
[otbn] Fully mux between model and RTL

The RTL implementation of OTBN didn't get disabled when the model was
selected due to an error in an earlier refactoring by me. Fix that, and
go the safe route to fully mux the start and done signals.

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

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