rstp: STP/RSTP disabled ports should not execute learning or forwarding actions.

      There is a difference between a port with STP/RSTP protocol enabled and a
      disabled role and a port which has a disabled role because STP/RSTP is
      not active. This commit ensure to make such distinction.

      Standard 802.1D claims that the Topology Change state machine (17.31)
      treats a Port as no longer active when it becomes an Alternate, Backup,
      or Disabled Port and stops learning from received frames.