rstp: Refactor rstp_port_set_administrative_bridge_port__().

rstp: Refactor rstp_port_set_administrative_bridge_port__().

Invoke move_rstp__() in rstp_port_set_administrative_bridge_port__()
if port is not initializing.  This is necessary in a test that checks
that a Port becoming alternate/backup/disabled stops learning and
forwarding.  The move_rstp__() call is necessary to immediatly disable
learning and forwarding on that port.  Without this, the test fails.

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