[otbn] Refactor controller state and error logic
[otbn] Refactor controller state and error logic

Drop the seperate `state_raw` using an assign to compute the final
`state_d` in favour of including all logic within a single
`always_comb`.

Signed-off-by: Greg Chadwick <3e4c3d6d87c017d1a2fa161477653b4039c6b2fb@lowrisc.org>

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