[mask_rom] Fix error in OTBN driver
[mask_rom] Fix error in OTBN driver

Fix the hardened check at the end of `otbn_dmem_zero`.

Signed-off-by: Chris Frantz <d3397dc265da044816f611610217f391e4e7ef39@google.com>

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