Before, the output data would be maintained after to valid_o goes LOW. this causes two problems
- main problem: if there is only single output word, rx_first_p1_o and rx_last_p1_o are
  maintained HIGH, they are generally expected to be pulses
- side problem: the data is maintained - it is OK, but it is supposed to be valid anyway
  only when valid_o is HIGH
So, now the data_o is set to zeros when valid is not HIGH. this should prevent
anyone from relying on the fact that data is maintaned after valid_o goes LOW and
it makes rx_first_p1_o and rx_last_p1_o to be pulses