[keymgr/dv] Fix keymgr_cfg_regwen
[keymgr/dv] Fix keymgr_cfg_regwen

Update scb to predict CSR `start` correctly

Signed-off-by: Weicai Yang <4fa8c7762ee4da95e3dee7dae2c3263ea16637f9@google.com>

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