[BRIDGE]: adding new device to bridge should enable if up

One change introduced by the workqueue removal patch is that adding an
interface that is up to a bridge which is also up does not ever call
br_stp_enable_port(), leaving the port in DISABLED state until we do
ifconfig down and up or link events occur.

The following patch to the br_add_if function fixes it.
This is a regression introduced in 2.6.21.

Submitted-by:  <Aji_Srinivas@emc.com>
Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>