This patch improves the device DNA reading FSM a bit.
It adds a (more interesting) value for use in simulation.
It avoids the Vivado complaint that 'WARNING: [Synth 8-3332] Sequential element (dna_reader/FSM_onehot_fsm_state_reg[3]) is unused and will be removed from module ipbus_device_dna_us_usp.'
It cleans a few things up (although this point can be debated).
This patch improves the device DNA reading FSM a bit.