wrsw_nic/nic_tx_fsm: reorganized FSM and some signal assignments

- removed declarations of unused signals
- reduced padding_size to bits needed for padding possible runt Ethernet
  frames
- simplified signal assignments and conditions in FSM (e.g. no need to
  check if needs_padding = 1 and padding_size /= 0, because when former
  is true, the latter is always true too)
- moved assignment of outputs validated with other outputs outside FSM
  to simplify the logic (fake rtu decision)