- removed ARB_DISABLED state, instead FSM is hold in ARB_START_SCAN
  unless module is enabled
- removed declarations of unused signals (granted_desc_idx, desc_idx_d0,
  check_count, stupid_hack)
- moved assignment of outputs validated with other signals outside FSM
  to simplify the logic (txdesc_current_o/rxdesc_current_o are validated
  by desc_grant_o, and stored in internal registers of nic_rx_fsm and
  nic_tx_fsm)
- rewritten ARB_WRITE_DESC state, so that data output is driven by
  desc_subreg counter instead of separate counter in case statement.
- more exotic regnums in marshalling functions (nic_descriptors_pkg) to
  minimize logic required to output correct data when needed in modified
  ARB_WRITE_DESC state