stp,rstp: disable learning and forwarding in STP/RSTP disabled state.

stp,rstp: disable learning and forwarding in STP/RSTP disabled state.

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treats a Port as no longer active when it becomes an Alternate, Backup,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.

Signed-off-by: Daniele Venturino <daniele.venturino@m3s.it>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>