network-bridge: Fix do_ifup in the case of ${bridge} != ${netdev}

network-bridge: Fix do_ifup in the case of ${bridge} != ${netdev}

On RHEL5.2, ifup ${bridge} fails if ${bridge} != ${netdev},
because RHEL5.2's ifup ${bridge} runs the following sequence:

1. Search CONFIG that has the same mac address of ${bridge}.=20
   ifcfg-${netdev} is found.
2. Run "ip link set dev ${netdev} up".
   # ${bridge} is expected.
3. Output "Failed to bring up ${netdev}."
   Because ${netdev} does not exist.

Thus, do_ifup() should not use ifup if ${bridge} != ${netdev}.

Signed-off-by: KUWAMURA Shin'ya <kuwa@jp.fujitsu.com>