[otbn, dv] Assertion fixes in otbn_idle_checker
[otbn, dv] Assertion fixes in otbn_idle_checker

This commit adds minor assetion fixes in otbn_idle_checker as they were
causing false failures.

Signed-off-by: Prajwala Puttappa <d42da9c1e7c0c7ddf8c0244ff16e57febc9395a0@lowrisc.org>

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