Just an addendum - if you think that it is ok to stop the motor in any phase, then the 8 stop states can be empty in the switch statement - either just:

switch (state & 0x0f) {
    case FWD_STOP_S1:
    case FWD_STOP_S2:
    case FWD_STOP_S3:
    case FWD_STOP_S4:
    case BKW_STOP_S1:
    case BKW_STOP_S2:
    case BKW_STOP_S3:
    case BKW_STOP_S4:
        // Nothing to do - just waiting for a running state.
        break;
    case FWD_S1:
    .
    .
}
 

or doing:

switch (state & 0x0f) {
    case FWD_S1:
        .
        .
        break;
    .
    .
    default:
        // Stopped - just wait for a running state.
        ;
}
 

Just make sure that all processed bits of the state variable gets processed or normalized, so that you can't get into an invalid state that locks up your state machine.